Cassie Wilson can pinpoint the moment her daughter Melanie became her son, Tom. They were in the supermarket and Melanie, then two and a half, said: “I don’t want to be a girl any more – I’m going to be a boy, and I’m going to be called Tom.”

“That was that,” Cassie tells me. “I said: ‘Come on, then, Tom.’ He could have said he was called anything and I would have thought, great, fine, let’s get on with the shop.”

“I was wearing my Spiderman costume,” Tom, now five, remembers.

“Yes, you were,” Cassie nods.

But it didn’t end when they left the shop. “The next day, when Tom woke up, I said: ‘Morning, Melanie’ and he said: ‘I told you yesterday! My name is Tom!’ It just went on from there.”

Cassie and I are sitting at the dining table in her home in the north-east, while Tom charges about in a Batman costume, brandishing a sword. Cassie, 30, is a single mother to Tom and his sister Carla, who’s just turned two. Side by side on the shelves near us are two framed photographs: on the left is Melanie in a white dress, with a cascade of blond ringlets; and on the right is Tom, still the same bright blue eyes, but with a boy’s short hair.

Tom loves dressing up. “Normally as a superhero,” Cassie says.

“Batman and Superman,” Tom adds. “And Wolverine!” He also likes to play cowboys or policemen with his best friend, Charlie. “Sometimes we arrest people. Remember when we did it yesterday to the dog?” He grins. “He wasn’t putting the ball down.” He shows me his bedroom. There’s his treasured Playmobil pirate ship, his Marvel poster featuring Ironman, Captain America and the Hulk, and his pencil case shaped like a football boot.

Cassie says she knows that Tom is very young to be transgender, but he is just one of a striking number of children who have been referred to the Tavistock and Portman NHS Trust’s Gender Identity Development Service in London over the past five years. A multidisciplinary clinic where all British transgender children are assessed, the service has seen referrals increase by 50% every year, from 97 new cases in 2009 to 697 in 2014. Young children such as Tom are unusual, but not rare: last year alone, close to 100 of those referred were aged 10 or younger.

Cassie tells me this was clearly “more than just a little phase”. Until that trip to the supermarket, Melanie had always been “quite tomboyish – I hate that phrase now”. But ever since that day, he has been Tom consistently, never faltering, even though at first he still dressed as a girl. But he always wanted to be called Tom, and to be referred to as “he” and “him”. If anyone used a female pronoun, he would get furious.

“On numerous occasions we would have to leave the park because someone would say: ‘Let that little girl have a turn.’ We’d get out of the park and he’d absolutely sob his heart out. It was the end of the world. He’d say: ‘Mummy, why can’t people see that I’m really a boy?’ This was before he was three.”

He still has girls’ clothes in his drawer. Cassie says he can choose to dress as a girl if he wants, but he never does any more.

When Cassie took three-year-old Tom to the barber for the first time, she wept. “That was the final thing. If I let him get his hair cut short, that was me accepting he is a boy.” The hairdresser was bemused. “I was crying and I had this little boy with me who had hair down to his arse. She asked him: ‘Has your mummy never let you get your hair cut?’ And he loved it, because she thought he was a boy with long hair.” After that, Tom never got mistaken for a girl, and became much happier.

“Granny still calls me ‘she’. My teacher still calls me ‘she’ sometimes.” Tom scowls as he nibbles a chocolate-covered rice cake. He’s about to go into his second year of primary school, where he’s fully transitioned: a male pupil. While the teachers have done everything possible to make things easy for Tom, the staff who have known him since nursery often get confused, because he was enrolled as a girl. “It used to make you upset but now you’re more grown up,” Cassie tells him, deliberately.

Like most parents of transgender children, Cassie learned about gender dysphoria from Google. Before she’d heard of the Tavistock or even visited her GP for a referral, she found Mermaids, a support group for transgender children and their families. “The Mermaids website is quite negative,” she says. “Every time I went on it, it made me worry… I knew from reading other people’s posts that I was going to struggle with my GP, because it seems to be a standard thing that they say no, we don’t believe in this.” She went to her GP armed with a printout of the NHS website’s page on gender dysphoria, prepared for a fight. Her doctor told her that she, too, had children, and that she needed to be tougher: tell her she’s a girl, and that’s the end of it. Tom was initially referred to child and adolescent mental health services, and then the Tavistock. He’s having annual appointments there, with his second coming up in a few weeks’ time.

***

No one can agree on what causes gender dysphoria, why so many children now say they are experiencing it, or even what it is, exactly: the idea that some people are just “born in the wrong body” doesn’t do justice to the range of feelings transgender people express. Gender itself is difficult to define, with a mix of social, medical and individual interpretations. The little research that exists on gender dysphoria is patchy and often fiercely contested. Some claim it has a biological basis; others argue it is a psychological state. Some say it must be genetic or created in the womb; others see it as learned behaviour, or a combination of nurture and nature. Whatever it might be, for the families of transgender children it is undeniably real.

“Tom is telling me what he wants and this is who he is, so I have to respect that even though he’s five. He knows his own mind,” Cassie says. “It could be that he changes his mind and goes back to being Melanie. But I can’t see it happening.”

This is biological, definitely. It’s too widely pathologised as being a psychiatric issue, when it’s not

The available evidence suggests most young children with gender dysphoria will not go on to be transgender adults. The Tavistock quotes research claiming that, while the majority of post-pubescent teens with gender dysphoria continue into adulthood with the same feelings, only 16% of prepubescent children do. (This figure comes from taking data from several different studies, with a total sample size of 246 children.)

Transgender children now have the opportunity to delay puberty by taking hormone “blockers” that inhibit the development of their secondary sexual characteristics, such as breasts, periods, a broken voice or facial hair. They can stop taking the blocker at any time and their own puberty will kick in, albeit at a later stage. But if they choose to proceed with the transition, they can then take cross-sex hormones from 16 years old, which will bring on the puberty of the gender they want to be. After they turn 18, they can opt for sex-change surgery through the NHS.

“Hand on heart, I do think that Tom will be trans when he’s older. He talks about ‘when I start having medicine to make me grow a willy’,” Cassie says. But she is making sure he knows his options are always open. “When he’s 18, he might be really mortified that this ever happened. But the message deep down is still going to be the same: it’s OK to be the person that you feel you are, and if that changes, that’s all right as well. You want your children to be happy.”

Meanwhile, the NHS is proceeding carefully with this explosion of child transgender referrals. Some parents’ groups claim it is too slow, too cautious and putting the wellbeing of young people at risk. “Medical intervention is very important, especially for teenagers who are already in puberty,” says Susie Green, chair of Mermaids and mother of a trans daughter. “It’s absolutely vital. If they feel their body is changing against their will, that’s when we get a lot of suicidality, self-harm, lots of young people talking about wanting to be dead. We’re trying to help the parents to help their children to get to a better place, and a lot of that is about navigating services and getting timely intervention – which unfortunately is not really happening as much as it should.”

Green became involved with Mermaids 16 years ago, after she got in touch about her daughter, Jackie, who was then six. “This is biological, definitely. It’s too widely pathologised as being a psychiatric issue, when it’s not,” she says. “The level of understanding within the NHS is appalling. If you’ve got a child who’s suicidal and self-harming because their body is changing against their will, nothing is done to fast-track or deal with that need. That’s why we’ve got families going abroad.” (This tends to be to the US, Germany and Holland.) Green reels off shocking figures from a 2014 study by the mental health charity Pace which surveyed 2,000 young people with gender issues: 48% attempt suicide, 58% self-harm. “It’s really common.” She pauses. “You can see why we’re worried.”

***

In the back garden of her Northamptonshire home, eight-year-old Julia King leaps on the trampoline with her 12-year-old brother, Dylan. She’s tiny and delicate, a pink hairband just holding her shoulder-length, white-blond hair from her face, her pink-framed glasses somehow staying put.

‘When will I grow into a girl?’ Julia started asking at the age of four. Photograph: Sophia Spring/The Guardian

I’m in the living room, drinking tea with Julia’s parents: Annie is on the sofa, Daniel lying on the carpet on his side, propped up by a cushion. They finish each other’s sentences as they tell me what can happen to transgender children like Julia.

“Depression. Isolation,” Annie says.

“Self-harming. Substance abuse,” Daniel says.

“Suicide,” Annie adds. “They kill themselves, you know. And I want a happy daughter, not a dead son.”

Like Cassie, the Kings have learned a lot about gender dysphoria from Mermaids. The whole family has taken part in the organisation’s residential get-togethers, where families meet and share information. Last time, Annie found out that Julia was telling people she was only there in support of her transgender brother.

“She’s cheeky. She’s funny. She’s quick” – Annie snaps her fingers – “very quick. She’s into everything. My Little Pony, Moshi Monsters. Her friends. Parties, hair, nails. Clothes! She can change her clothes three, four times a day.”

Julia was born Callum, but as soon as she could talk, she said she was a girl. From two or three years old, she refused to answer if people called her by her birth name.

“She used to draw herself as a girl as soon as she could hold a pencil,” Daniel says. “I’ve never seen her draw herself as a boy, ever.”

Unlike Tom, there was no single turning point when Callum became Julia. “It just – from a little, little age – happened,” Annie says, brushing her pink, dip-dyed fringe from her eyes.

Daniel nods. “It evolved.”

At nursery, she was always in the dressing-up box, choosing the girls’ costumes. When Julia was given her first dress, at two and a half, she would wear it to bed. “She wore it until it was falling apart. We had to buy a new one because she was growing out of it.”

Julia still had a boy’s haircut at the time, and they didn’t let her wear her dresses outdoors. “She would ask: ‘Why? Why?’ And I’d say to her: ‘People aren’t used to seeing boys in dresses.’

“When she was around four, she started asking questions. ‘When will I have a baby in my tummy? Why do I have a winky when I’m a girl? Will I have boobies one day? When will I grow into a girl?’” Annie winces. “These aren’t normal questions. Dylan never asked them.”

Dylan comes in. He’s tall with square shoulders, his hair shaved at the sides with a floppy quiff on top. Julia’s having her picture taken and he’s not interested in watching her pose.

What’s Julia like as a sister, I ask. “Just a sister,” he shrugs. “I don’t see her as a boy or a girl. She’s just my sibling.”

Julia never wanted Dylan’s little cars, his action figures or his Star Wars toys. Her Christmas list, aged three, was full of Barbies. Annie and Daniel had never heard of people who were transgender at that point, and didn’t know how to respond.

“I struggled, initially,” Daniel says. “I had a big issue, not with her, but with the fact that I’ve got a boy and I’ve got another boy who wants to be a girl.” Daniel works in uniform, in a very conventionally male world. “I wasn’t ashamed, but there’s something in you, your pride or something, where you think, this isn’t what men do.” Now, he says, he’s incredibly proud of his daughter. When they changed her name by deed poll a few weeks ago, he toasted her with champagne.

Julia’s grandparents have found it harder to accept. “My mum said: ‘You wanted a girl, that’s why this is happening’,” Annie says. She thinks it’s a fad. “She’s said: ‘This is all some American thing.’”

The energy has to go into changing how they are seen by their peers, not into physical intervention

When Julia was five, social services rang. “They said we’ve had an anonymous call, that you’re making your son Callum wear dresses,” Annie tells me. “I went ballistic. I said, ‘You are more than welcome to come to my house, because when you go in Callum’s bedroom…”

Julia comes in with a plate of cocktail sausages.

“…he’s got pink walls, he’s got princess castles, he’s got Barbies…’” Annie continues.

“He?” asks Julia, indignant.

“…he wears princess dresses.” Annie throws her hands up. “I don’t care what anybody thinks.”

“Who’s he?” Julia frowns.

Upstairs, Julia’s bedroom is meticulously tidy. Her shelves are stacked with boxes of loom bands, a Shimmer ’n Sparkle nail kit and books with titles such as Love Lessons and The Illustrated Mum. She sits on her bed, twisting foam flowers on to a pipe cleaner to make a ring.

“When people say ‘he’ it makes me feel nervous,” she says gently. “That they’re going to forget and call me it from now on. That it might go back to how it was. All my friends call me Julia now. They never say C-A-L-L-U-M.” She can’t bring herself to say it.

But then she says something I wasn’t expecting. “Sometimes I say to my friends – because they were born a girl – ‘Would you be a boy for a day to see what it feels like?’ And they say yeah, but I’m like: ‘I don’t have to because I’m both.’”

Do you really think you’re both, I ask, or do you think you’re a girl?

“I’m both. It’s all right if I’m both. I want my mum to take me to football lessons.” But then the next minute she says she never feels like a boy, even though being a girl can be tough. “When you’re a boy you fall out and then the next day you’re friends, but with girls you fall out for months. It’s easier when you’re a boy.” She talks me through her collection of figurines. There’s Slimer from Ghostbusters, Raven from Teen Titans Go, Predator, and Anna from Frozen. “I don’t really like Frozen. I like horror films,” she grins. “My favourite film is Chuckie. For Halloween, I’m going to be Chuckie’s bride.”

Back downstairs, I tell Annie and Daniel what Julia said. “She knows the trans world better than we do,” Annie says. “Will there always be confusion in her mind?” she asks herself.

But is it confusion, or can you just be both?

“She can be whatever she wants to be, however she sees herself. She’s just our little girl at the moment.”

Julia is enrolled at her Catholic primary school as Callum. The school had suggested she finish the year as Callum and come back in the autumn as Julia, and the Kings had planned a big coming-out party for her over the summer – but Julia made it happen more quickly, insisting on wearing a skirt to school earlier this year.

“I said, ‘Do you really want to wear this?’” Daniel says. “She said, ‘Dad, I’m scared, but I want to because I have to.’”

“When she does things like this, you realise she doesn’t have the choice,” Annie adds. “She will put herself in the most awkward position. We don’t hold her back.”

They won’t hold her back when it comes to medical intervention, either. “We need to get these hormone blockers,” Annie says. “I wouldn’t want people to think that I’m pushing for something she doesn’t want, but I’m very aware of those years that could damage her future if she’s going to live the rest of her life as a woman. If we can get them at 10, 11, 12, brilliant. All the decisions are hers. We make her aware of what is out there, what they do, how she takes them, what she can have – even operations.”

At a Mermaids residential meeting, the Kings learned how other countries treat transgender kids. “They said that places like Germany and Holland are ahead of the UK in relation to giving the blockers,” Daniel says. “And I told the Tavistock, if there’s any blocking [of Julia’s treatment] I’ll remortgage and I’ll take my child over straight away. There’s no way I’ll stop my child’s happiness because of your restrictions.”

“We have made that quite clear to everyone,” Annie adds.

‘When he’s 18, he might be really mortified this ever happened. But hand on heart, I think he will be trans,’ says Tom’s mother. Photograph: Sophia Spring/The Guardian

At the Tavistock, they are taking a more cautious approach. I meet Dr Polly Carmichael, the consultant clinical psychologist who leads the Tavistock’s Gender Identity Development Service, in her office in a leafy corner of Belsize Park, north London. A stone Sigmund Freud sits deep in thought in a corner of the car park. Carmichael is concerned that physical intervention is being seen, unrealistically, as a panacea.

“When the idea of the blocker being available to younger people was being pushed forward, I think that inevitably – understandably – there were quite simplistic arguments that if you have the blocker then all the problems disappear. In our experience, all the problems do not go away.”

The blocker can be therapeutic in itself, because it takes away the anxiety that comes with going through the “wrong” puberty. It also gives young people much-needed time and thinking space. “The idea was, if you could reduce that distress, then there would be room for young people to really explore the less reversible interventions: cross-sex hormones,” she explains. “But there’s also a lot of pressure to introduce cross-sex hormones at a younger age. It’s currently at 16. For some, there’s a real wish to bring it down to 14.” When I ask who she means, she says Mermaids and the Gender Identity Research and Education Society (Gires), a transgender advocacy charity based in the UK. “Really big changes like that should not be considered outside proper research protocols. We just don’t have the evidence.”

Carmichael says it’s very important for young people to experience some of their own puberty. “The blocker is said to be completely reversible, which is disingenuous because nothing’s completely reversible. It might be that the introduction of natal hormones [those you are born with] at puberty has an impact on the trajectory of gender dysphoria.” Even though the idea of experiencing any “natural” puberty might horrify the Kings and the Wilsons, by inhibiting it completely Tom and Julia might be denied the chance to explore fully who they are.

The available evidence suggests that most prepubescent children with gender dysphoria will have a different outcome in adulthood, Carmichael says: “The most common would be one around sexuality, rather than gender identity.” In her experience, they are more likely to be LGB than T.

When Carmichael took up her role in 2009, it was rare for children to be socially transitioned in primary school. Now it’s becoming the norm. What are the implications if only a small proportion will end up as transgender adults?

“If a lot has been invested in living in a gender role, then, potentially, it is difficult for young people to say: ‘Well, actually I don’t feel like that any more.’ Parents rightly want to support their child. Parents report that many young people who do make a social transition are much happier, that they’re functioning much better – which is why there isn’t a right and wrong. It’s about that child being able to carry on with general development, be in school. If transitioning facilitates that, then that’s positive, but how do we keep in mind a diversity of outcomes?” She pauses. “It’s really hard, a real challenge.”

The answer, she suggests, might lie in understanding gender as a spectrum, not a set of binary categories. “It would be great if society were more open to a range of presentations. I think if we could break down some of the gender stereotypes around boys and girls being divided in school the whole time, then that would be positive.”

Consultant clinical psychologist Dr Bernadette Wren works alongside Carmichael. She’s concerned that gender is becoming increasingly polarised, at an ever younger age. “Some of these children are going into a school environment where they make gender choices all day long, and they shouldn’t have to,” she says. We discuss how pupils have to choose what uniform to wear, what books to read, what sports to play, even what stationery to use, and I think of Julia insisting on wearing her school skirt, and Tom’s football-boot pencil case. “It’s not that you want everyone to be androgynous and gender fluid, you just don’t want children to constantly have to make those choices. We need to preserve a space where children can explore their gender identity without having to make a commitment so early.

“The younger ones can really, really want to be girls or boys, and then they can give that up and their relationship to their bodies can settle down quite comfortably. If we can help some of those young people through adolescence, they might make a different choice later.”

What about the risk of depression, suicide and self-harm at puberty, cited by parents?

“It troubles me that parents of very young children are already in terror that their child is going to kill themselves,” Wren says. “The energy has to go into changing how these people are seen by their peers, not into physical intervention alone. You don’t want these kids to have to make themselves over at a very early stage because otherwise they’ll be tormented to death.”

But what about the young people who say that living in their body is itself a torment?

“We have shifted to make the treatment available earlier and earlier. But the earlier you do it, the more you run the risk that it’s an intervention people would say yes to at a young age, but perhaps would not be so happy with when they move into their later adulthood.”

***

The argument over whether to intervene early or wait and see how a child’s gender identity develops naturally is so polarised, with such potentially serious implications, that it can be baffling – terrifying – for the families caught in the middle. Whatever the correct balance, medical intervention alone cannot ensure that transgender children become healthy, happy adults. More than anything else, transgender children need resilience; even if their family accepts them, and however tolerant we are as a society, they will need enormous strength to manage the choices and challenges they will face as they grow up and form relationships.

The Kings have already had their first taste of those challenges. When we meet, it’s the day after Julia’s school disco – the first time she’s ever been properly dressed up for an occasion – and Annie’s telling me about it with tears in her eyes. “I watched her run in across the hall, and all the girls running to her. Last night she was one of them. And I knew how much it meant to her.” Julia danced with a boy called Luke, and they’re now boyfriend and girlfriend. “But my worry is, what’s Luke’s mum saying when he goes home and says: ‘I’m going out with Julia, Mum’? I don’t think society has come to terms with trans as far as parents letting their sons go out with a transgender female. They’re fine to let them play together, to go to school together, to be best friends. But relationship-wise? I’m not sure.”

Annie might not want Julia to know she’s worried, but Julia will learn the reasons for herself soon enough. “We’ve just got to be there, ready to catch, if anything falls apart.”

• Some names have been changed.