It's the most won-der-ful time…of the year. At least, it is for young audiences. Over the next month or so, theatres up and down the country will do bumper business – and all because, for a few weeks a year, they put children first.

Panto season is not just about restoring a theatre's coffers. It's about winning new recruits. Listen to the Royal Court's Playwright's Podcast and, whenever host Simon Stephens asks an interviewee about their first theatre trip, the vast majority remember an annual, seasonal family outing – to the local panto perhaps, to the ballet or maybe a musical.

Theatre changes in December. It's the one time of year when the young are spoilt for choice. When theatregoing becomes something we do en masse, as families, theatres (and theatremakers) start taking young audiences seriously. Suddenly, child-friendly shows storm the main stage. Silliness holds sway and family fare dominates. As January unfurls and cold classrooms refill, though, normal service resumes and grown-ups reclaim the theatres.

Children's theatre isn't taken seriously in this country

Last week, the director Sally Cookson told me that, if it were up to her, every major subsidised theatre would stage children's work all year round. There's a lot to be said for it.

Children's theatre isn't taken seriously in this country. It remains secondary. Beyond a handful of dedicated venues – the Unicorn and the Polka, The Little Angel and the Egg in Bath – kids' shows are only a sporadic presence. The biggest theatres in Britain offer one, maybe two, a year – and even then, usually only because it's Christmas.

Under Purni Morell, the Unicorn has done a lot to get top-drawer artists to make work for young audiences. There are many devoted children's companies too – Theatre Centre and Boundless, Travelling Light and Theatre Rites – who are always on the lookout to open up spaces for young audiences, be they in conventional theatres or outside of them.

Too often, though, Britain's big theatres shunt children's shows under education. The National and the RSC tour reworked Shakespeare into schools. The Lyric Hammersmith rolls travelling shows through its studio, but never contributes any of its own. When major theatres produce children's work – as the Royal Court did with The Twits, say – there's a momentary fanfare about the importance of kids, then it's back to theatre for big boys and girls.

Five seats are better than two. Family trips trump date nights hands down

There's a market for it. Kids' shows pop up in the West End come school holidays and half terms. Companies like Tall Stories and producers like Kenny Wax will move kids shows into empty theatres by day, confident of covering their costs. Family shows, meanwhile, are theatre's golden geese. Shows like Matilda, Wicked and Harry Potter keep the West End afloat and have done for years. Five seats are better than two. Family trips trump date nights hands down.

Theatregoing is a habit. Instilled in children early, it can last a lifetime. The same goes for not going. When we talk about barriers to access, this is a part of what we mean: reasons that people habitually stay away. One is that they just never went. As education cuts become entrenched, theatre's falling by the wayside in schools. The take-up for arts subjects is plummeting and technology's allowing recordings of theatre to replace the real thing. If we want the next generation of audiences, theatres have to start cultivating them early. Theatres have to challenge that themselves.

Theatres have to start cultivating audiences early

That's an instrumental argument though: children's theatre creates adult audiences. There's an intrinsic one too. We should treat children as audiences in their own right. They may not pay taxes, but they're citizens too, and if we truly believe that theatre has a role in civic life, there's no reason why that should start once adulthood arrives. If the National Theatre, for example, wants to be a National Theatre for Everyone, as the banners outside it proclaim, then its programme has to offer something for children. It does – but why not year round?

Why can't parents take their kids to shows after-school, just as adults go after work? Why aren't our public theatres addressing a significant part of the population all the time? How do theatres reach young audiences without reducing theatre to an educational tool? It has to be a pleasure in its own right; a space for reflection, entertainment and play.

Practically, it wouldn't take much. Empty stages exist and costs can be kept low. Shows have long shelf-lives as audiences replenish themselves year-in, year-out. All it takes – and this is no small ask – is a culture shift. Not just from theatres, who'd need to think outside existing models, but from parents and carers as well. We'd have to take theatre seriously, and young audiences too. Children's theatre should be for life, not just for Christmas.

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