‘Life is too short to stuff a mushroom”. Four decades on, the author Shirley Conran’s maxim for working women still holds true. She wasn’t talking only about time management, or indeed cooking, but about the hidden cost of living up to other people’s expectations.

If you’re madly busy, stop thinking you have to spend hours in the kitchen every day; just getting dinner on the table will do. Time is finite. Hours spent trying and failing to match some ridiculous feminine ideal just means less of it for something else.

It’s advice that never dates, which is why the mild backlash triggered by the writer Zadie Smith at the Edinburgh international book festival this week is in some ways surprising. Worried by the amount of time her daughter spends looking in the mirror, Smith said, she had told the girl never to spend more than 15 minutes putting on makeup because “your brother is not going to be wasting time doing this”.

To be clear, Smith wasn’t banning makeup, or saying any woman who has ever resorted to mascara is an empty-headed fool. Her regime is arguably far less strict than that of the former newspaper editor Eve Pollard, who according to her daughter Claudia Winkleman wouldn’t let her teenage children have mirrors because she wanted to teach them that appearance matters a lot less than being able to “read books, be funny, be clever, be chatty”.

All Smith was doing was pointing out that time is finite. Sure, slap some makeup on to make yourself feel confident in the morning. But life’s too short to spend an hour every day trying to contour (a Kardashian-inspired process of trompe l’oeil, using dark and light shades of makeup to create the illusion of a resculpted face).

Regardless of what they think of her methods, many mothers of daughters will share Smith’s unease about the time spent staring either into the mirror or into the virtual reflection of their smartphones. The issue isn’t vanity but the crippling anxiety induced in girls, and increasingly boys too, when fragile adolescent self-esteem hangs on the number of likes a selfie gets. Smith’s daughter is only seven, which means all this is presumably theoretical right now, but she is wise to start the conversation early.

The children of No More Boys and Girls, BBC2’s provocative documentary about a class of primary school children encouraged to challenge gender norms, were only seven too, but they had some alarmingly fixed ideas about what being a girl means: lipstick, dresses, being pretty. Heartbreakingly, one little girl described herself as “ugly”, a word that had the children’s parents wincing when shown the results.

It’s primetime telly, not real life, so of course the answers presented were simplistic and the neuroscience at best basic. At one point presenter Javid Abdelmoneim emptied a child’s bedroom of all stereotypically “girly” toys, as part of an experiment encouraging the girls to work with construction toys and develop their spatial awareness, while the boys were encouraged into more nurturing play. Poor Maisie, contemplating binbags full of princess dresses and Barbie dolls dumped on the landing, was understandably wobbly-lipped.

But her deprivation was only temporary and the gist of the programme hard to deny: not that there are no differences between the sexes, but that artificially narrow ideas of what boys and girls can do are ridiculously constricting and unhelpful to both sexes. As the experiment wore on, the girls’ confidence and academic expectations increased, while bad behaviour among boys – who had been encouraged to express and manage their emotions – declined.

Girls should feel free to play with robots and Lego if they want, just as boys should have the chance to play with craft kits and teddies – and anyone who thinks this goes against the grain of nature doesn’t know nearly enough seven-year-old boys.

Yes, they’ll turn absolutely anything into a gun, given half a chance (one friend relented when hers started chewing their breakfast toast into revolver shapes). But sleepovers when my son was that age would invariably involve a houseful of testosterone-fuelled little boys brandishing weaponry – until bedtime, when out would come piles of threadbare cuddly gonks, without which none of them could sleep. Little boys like guns and teddies, and shouldn’t have to feel ashamed of either. But the same is true of little girls too.

The point is to add options to children’s lives, not subtract them

Last week, the newly elected Tory MP Paul Masterton posted a snapshot of his three-year-old daughter’s first day at nursery. He was promptly criticised by an SNP councillor for letting the girl wear a pink coat while her baby brother’s T-shirt had trucks on it. How very dare he enforce gender stereotypes?

Yet according to her father, while Daisy Masterton does indeed like pink and Disney princesses, she also likes playing with cars, and Thomas the Tank Engine. The two are not mutually exclusive. “Girls can like glitter and cars; boys can like dolls and can like dinosaurs,” said the MP for East Renfrewshire, who admits this isn’t the issue he expected to be dealing with in his first few months in parliament.

But he’s right. Challenging gender stereotypes doesn’t have to involve running screaming from anything traditionally associated with one’s gender. Ban dolls, pink or glitter, and you just feed a desire for forbidden fruit.

The point is to add options to children’s lives, not subtract them. Kids encouraged to play with anything from chemistry sets to feather boas stand a better chance of being able to identify their passions for themselves – to grow into adults who understand that the sky will not fall in if they express emotion, or that liking shoes isn’t incompatible with a career in engineering. As Harriet Harman said at the height of the hysteria over Labour’s pink election battlebus, having a bright fuchsia iPad cover didn’t somehow drain her of feminism at a stroke. All that matters is achieving a balance, both in children’s lives and sometimes in the lives of over-anxious parents.

It’s no more healthy for teenage girls to spend hours obsessing over their faces than it is for boys to spend hours hunched over a console playing shoot-’em-up games, and in both cases parents need to intervene and make their children think about what else is being squeezed out of their lives. But 15 minutes a day of experimenting with eyeshadow, or getting a Nerf gun for Christmas, isn’t going to ruin anyone’s life chances.

And if adults are still groping through this minefield in some confusion, I wonder if kids aren’t quietly making rather less heavy weather of it. Charged with dishing out the temporary stick-on tattoos at a children’s disco recently, I noticed how strictly the kids seemed to distinguish between “girl” ones (hearts, flowers, princesses) and “boy” ones (superheroes, skull and crossbones). Jumble the stickers up, and they would still search diligently for the “appropriate” ones. But then the most alpha boy of the lot demanded a Snow White tattoo on his forehead, and suddenly, all the boys were clamouring for them.

They knew princesses were girly, and they knew they weren’t. And that’s precisely what made it an appealing game: they were confident enough to play around for a laugh with concepts that still make their parents anxious. Given half a chance, I suspect the kids will work it out. It’s the rest of us who need to grow up.

• Gaby Hinsliff is a Guardian columnist