Quentin Blake’s new illustrations for the covers of Matilda, celebrating 30 years of the book, provoke a mixture of emotions, simultaneously empowering and bittersweet. On the one hand, it is glorious to see Matilda at 30, taking on the role of explorer, astrophysicist, or running the British Library. We need to see that happening. It makes us all reflect on what an intelligent, imaginative and industrious girl ought to be able to be. So why is it also just a tiny bit unsettling? Why does a part of us not want to know what Matilda has become? Somewhere in our heart of hearts we never want Matilda to grow up – we want her to be like Peter Pan, eternally young.

For one of the most exciting things about Matilda as a character is that she can obstinately think and imagine her way out of impossible situations, a peculiarly child-like characteristic, and perhaps we worry that if she grows up she might lose that magic.

And this reflects a tension inherent in all children’s books, which are almost always about growing up. My How to Train Your Dragon books are narrated by Hiccup as an old man looking back on his childhood, just as all adults reading children’s books aloud with their child are carried back into remembering what it is to want magic to shoot out of your fingers, to push at the back of a cupboard and find Narnia behind it, to look once again through the cool, clear eyes of a child, when a belief in the impossible is as natural as breathing. This is on the one hand exhilarating, going back to a time when all options are open, even the impossible ones, but at the same time a little melancholy, for the adult is looking back with experience. They know how the story ends, for them, at any rate, and for their generation.

Looking at those Matilda covers makes us question how far we have come as a society in 30 years. That’s where the bittersweet part comes in. Not far enough.

When I was a child, Matilda had not been written yet (I’d have loved her when I was nine) but I clung on hard to heroes like her. Pippi Longstocking, Meg in A Wrinkle in Time, the unnamed but powerful and headstrong girl in The Magic Finger, a Roald Dahl story that was a precursor to Matilda. I clung on because there simply weren’t enough strong girl heroes out there for me, in books or in films, or in real life. I was frustrated by the fact that there was no Jane Bond, no female head of a major company, so few women in the Houses of Parliament. But in my Matilda-like, hopeful, ebullient way, I assumed that the future we were travelling towards would be very different from the present.

Now that I’m actually living in that future I have to say the rate of travel has been considerably slower than I expected. The recent photograph of Theresa May creeping behind the massed backs of the suited men of the EU is for me poignant because it is a vivid illustration of how things have not changed as quickly or as radically as that long lost nine-year-old so confidently hoped.

We do have far more female heroes in children’s books who dynamically direct the action rather than passively letting it happen to them: Ruby Redfort, Katniss in The Hunger Games, Feo in The Wolf Wilder. In my new book Wizards of Once: Twice Magic, the girl hero, Wish, is the one with the truly great magical powers. My books are read by equal numbers of boys and girls and every time a nine-year-old boy in my signing queue says that his favourite character in my books is a girl, I do an internal high five. Yes! Progress is being made.

Adults can learn from children’s lack of prejudice. We can learn from their hopefulness, their endless questioning, and their belief in the impossible. Children are the most creative people in the world, because they don’t know the rules yet. And we’re going to need every single ounce of that creative intelligence to come up with solutions to our current political and scientific challenges. What with one thing and another, the grown-ups have made a gigantic howling mess of things.

So go, Matilda, go! Be an inventor, be an explorer, be a brilliant politician (we really, really need one). But stay young, stay obstinate and, for goodness sake, stay magic. We need your bravery and your brains and your bright eternal youth.

• Cressida Cowell is the author and illustrator of the How to Train Your Dragon and Wizards of Once books