Lia Hegarty, a 10-year-old transgender girl, loves theater and wants to be a zoologist.

For the past three years, she has legally changed her name from Liam to Lia, wears girl’s clothes and will soon take hormone blockers to ward off puberty. “I say I am a girl stuck in a boy’s body,” she tells FRONTLINE in “Growing Up Trans,” which airs Tuesday June 30 on PBS and online. The documentary looks at a new generation of younger children who have gender dysphoria and how new medical options are helping them.

Lia’s mother, 40-year-old Christy Hegarty from Durham, New Hampshire, shares the remarkable story of her daughter’s transition.

Christy Hegarty with Molly and Lia (right) in 2015. Courtesy of Christy Hegarty

I can’t lie. It’s been a really surprising and wonderful experience. Lia has been able to transition in a gradual and thoughtful way, on her terms.

It wasn’t until I got her report card in the second grade that I realized she decided to tell the school she was ready to change her name. The report said, “Lia, otherwise known as Liam, did really well in class this year.”

That’s part of the beauty of Lia’s story: Everyone was able to give her the room to figure it out for herself, and to let her make her own decisions. Last fall, we officially changed her name and even changed her birth certificate.

She became enamored with wearing dress-up clothes before preschool. I remember her being very interested in wearing her sisters’ clothing and asking, “Why can’t I dress up like Catie and Molly?” At the time, we answered, “Well, because you are a boy... and they are girls.”

We didn’t know if it was a phase and we were not overly concerned. Lia was a cute kid with a strong personality and we were not afraid of letting her explore her fashion options. My husband, Boyd, and I had considered maybe she would be gay or effeminate, but we didn’t panic and we were OK to let her become whoever she wanted to be.

We just wanted her to be a happy and healthy child!

We moved from Indiana to New Hampshire when Lia was 4. She was still wearing boys’ clothes and playing with trucks through preschool, but she really wanted girl things. We started buying her Barbie dolls and girl’s costumes.

For awhile, Lia was happy wearing pink dress shirts outside and princess costumes at home. Family and friends were curious, but no one made us feel judged. However, It was when she started kindergarten, that we saw the dysphoria start, the depression and anger. It was clear she couldn't be comfortable anymore.

Lia is the most mild-mannered, peaceful warrior, a deep thinking, soul-searching child. The minute she begins talking, it’s just pure love. But we could see the frustration building in her.

She would say desperately, “Why can’t I just be a girl, Mommy?” At just 5 years old, she would look in the mirror in the bathroom and say, “I am a girl in my head and heart.”

Lia as Liam at 2 and Catie, 5. Courtesy of Christy Hegarty

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Around that time I saw a TV show about transgender children. By the end of the segment, I was bawling my eyes out. It was so clear to me. I knew it wasn’t a phase. I got online immediately and stumbled across a blogger, and she put me in touch with a doctor in Boston. This was the beginning of our journey and the search for support and guidance.

I had never knowingly met a transgender person. I didn’t understand that gender identity and sexual identity are not connected. I had a lot to learn.

Getting Lia dressed for school was always a challenge and she would come home and start dropping her clothes up the stairwell then get into some sort of princess costume. Soon it became clear. It was stupid to make it all about a shirt and a pair of pants. We did our best to follow her lead.

Lia started first grade in boys’ clothes, then one day, she decided to wear some girls clothes we purchased for her to school, dark jeans and “girly” shirt. I didn’t prep anyone. No one said anything.

The next day, she was confident enough to wear a skirt and a pink shirt, so I thought I should probably call the school. The principal offered to meet her off the bus, for support, but I told him that wasn't necessary.

Pumpkin picking time: Catie, 8; Lia, 5; and Molly, 2. Courtesy of Christy Hegarty

The guidance counselor led “circle time” discussions about what it means to be gender variant, presenting it as about kids’ likes and dislikes. Susie likes to play baseball, Johnny likes to wear shorts everyday .. and Liam likes to wear girl’s clothes.

Most of the kids were okay with it and some asked questions. Lia was so confident and self-assured. She powered through. But when she came home that night, she was really sad — she told me that the principal had asked everyone over the loud speaker not to make fun of people who may make different choices. When she realized people had laughed, she was disappointed.

But it didn’t break her spirit. That night at an event she went back to school with us and it was our debut as the parents of a boy who likes to dress in girls’ clothes. Her bravery at that time was overwhelming to me.

There was a time when I thought we were the only people in this situation, but now we know we’re not alone. The Facebook group —Parents of Transgender Children — has been a huge support. Five years ago there were maybe 500 members worldwide. Now there are 2,134.

It’s given me the space to be able to talk with other families in the same situation and find support. We have also found Camp Aranu’tiq, a camp for transgender and gender variant kids, to be a safe and supportive space for our child.

We have had an appointment with an endocrinologist and in six months to a year, we will start looking at puberty blockers. That’s important to Lia. We feel it’s the safest and best way to make room for her choices over the next five to 10 years.

Lia, 10, all dressed up for her fourth grade recognition ceremony where she gave an “end of the year” speech to her classmates and their parents. Courtesy of Christy Hegarty

The puberty blockers like, Lupron, don’t scare us. It’s a way to buy Lia time.

But we are entering a challenging period for most kids — middle school — and while she’ll be stopping puberty, her friends will be moving on and we are preparing for her to experience dysphoria again.

At this point, Lia will tell you she wants to be a REAL girl, to have the anatomical parts match who she is on the inside. I am 95 percent sure she will continue to grow that way. But it’s a big decision. It's her decision, and all we can do is continue to learn and help guide her.

As for concerns about using hormones in the future, I can't help but compare it to children who have a life-threatening illness. We see statistics of high suicide rates and violence against transgender people. If I can give my child a chance to live the life she so desperately wants to live, we might risk a side effect or two. But we go cautiously and have hope and faith. That’s all we can do.

We love seeing Laverne Cox on the TODAY show and on magazine covers. We also appreciate and respect that Caitlyn Jenner is helping to educate by telling her story. We are happy to know things are moving forward. There is hope.

The Hegarty family in 2013: Christy and Boyd with Catie, 10; Lia, 7; and Molly, 4. Courtesy of Christy Hegarty

But it doesn’t squelch my fears for our daughter: It only takes one person to damage my child.

My message to other parents in this situation is, please, know that you are not alone. This is real and there are many places to go for support.

For me the biggest struggle was being patient. Like every parent, I wanted to fix things and make them right. I had to sit back a bit and let things unfold, because it’s not my story, it’s Lia’s story. And it’s been my joy to give her the unconditional love she deserves.

--As told to NBCNews.com contributor Susan James

To learn more:

The Trans Youth Equality Foundation

Gender Spectrum

Welcoming Schools

“The Transgender Child”