SOME TIME AGO I gave a talk in a theatre. There were many questions - particularly from children, the most perceptive of questioners. Then a man raised his hand and asked: “Is it the case that you start off by writing for children, then you grow up and start to write for adults?”

And I had to say that I had taken the opposite route. I began to grow up as a writer when I started to write for the young. It wasn't a choice. It happened by what felt like a kind of magic. I had been writing for years, an intelligent grown-up writing stories for intelligent grown-ups.

Then as I ambled through Reigate one summer's day a new story, Skellig - about a boy who finds an ailing angel in his garage - took flight inside my brain. As soon as I began to write it down, I knew it was the best thing I had done until then. I knew it was the culmination of everything I had done before, and to my amazement and excitement, I realised that I was writing a book for children - for readers who know they don't know everything yet, whose minds are flexible and able to explore all kinds of possibilities. For readers who are, like the best books, only half-civilised.

I love being a children's writer. Don't believe the pessimists who say that children don't read any more, that their minds have been stunned by screens and PlayStations and iPods. It simply isn't true.

Watch children reading in a library or a bookshop. Look how engrossed they are. Go to a children's book event at Edinburgh, Cheltenham, Hay. Listen to the questions and the speculations about subject, form, language. Children form an engaged and responsive readership, and the children's book world is a place of huge creativity and experimentation.

Wander through a children's book department. Long books, short books, comic books, picture books, poetry, plays. Books dense with a hundred thousand words, books without a single word at all. Books written from the point of dogs, bears, aliens, rats. Books that turn to sculpture, books that squeak, books that flash. Books whose manner and whose form would seem, to many adults, difficult, challenging, simply too weird.

Writing for children reminds us that books are literature, that all books stand on shelves in relationship with the books of Dante, Dickens, Dostoevsky; but that they are also in deep relationships with stories murmured at dusk to children who are about to sleep, with stories oohed and aahed and sung to babies who appear to respond and listen, even though they have no apparent way of understanding a story at all.

It reminds us that storytelling is ordinary and everyday, but that it lies at the heart of our culture and history. Our brains are wired for telling and listening to tales. All of us have some kind of innate love and understanding of rhythm and form.

Creative writing, like all art, is a kind of play. Yes, it's a difficult and serious and grown-up activity, but it's also a game. The best question I have been asked about the creative process came from a girl of about 12: “How do you turn all the mess in your head into straight lines on the page?”

The answer, I think, is that you aim for the apparent perfection of beautifully bound books and printed pages, but you start by plunging into the mess. Writing is a scribbly, doodly, imperfect, playful process. Don't get stuck, don't give up. Trust your imagination and your pen. Just write. Fill the blank page with scrawl, with rubbish, just like a five-year-old would.

Extract the images and details from the mess and work with them, grow them into sentences and paragraphs and pages. Plot and plan, but don't let the story get stifled and trapped by the plan. Don't worry about what you should not put into a book for children. If you start with limitation, with too much control, your story will become weakly unconfident.

Write the best possible story in the best possible way. Write in your way, which will of course be shaped by your life, your experiences, by all the books you have loved. Be as simple or as complex as the story demands. Be guided by the minds and worlds of your protagonists.

Don't try to teach lessons or send messages. Aim to charm and challenge your readers. Allow the story to grow like a living thing. Allow it to surprise you and challenge you, just like a growing child would. Stay with it. Don't give up. Write a page, then another page; a chapter, then another chapter. Write and rewrite until you confidently write The End. Then start scribbling a mess again, and finding fragments of the next book.

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