The silvery, petite and very pretty sixtysomething woman chatting to me over coffee is a Dame of the British Empire, but there is absolutely nothing of the grande dame about Jacqueline Wilson, whose heart-rending but cheerful stories of vulnerable children sell in their millions in more than 30 languages, mostly to children aged eight to 14.

We are at a hotel in London's Waterloo to talk initially about Wilson's best-known character, Tracy Beaker. About to get a new lease of life as the heroine of a videogame, Tracy is a feisty, troubled, troublesome, but profoundly lovable child in a care home. She narrates several books and has featured in a number of BBC series. I tell Tracy's creator that I had grave misgivings about fine books (by a former Children's Laureate with a brief to encourage reading, no less) being reduced to a videogame – until I saw the game in action and realised how surprisingly bookish it is.

"Yes, I had doubts, too," she agrees, telling me that she sees herself as a sort of protective foster-mum to Tracy. "I had heard that videogames were often violent and, given Tracy's volatility when she's distressed, it would have been easy to hijack her." So she insisted on a full consultancy role. The result is a game which requires users to read whole sentences and get involved in shaping the narrative by making choices for Tracy. It is, we agree, likely to nudge young game addicts towards reading the books. Bravo.

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Wilson hopes that Tracy Beaker: The Game will entice more boys, too. She has no idea ' how many boys read her books, but at signings – and in her huge postbag – 90 per cent of her fans are girls. "Although my books are read as part of school classwork, too, so boys are involved there," she says.

Quietly spoken, elegant and articulate in her unassuming way, Wilson has published more than 90 books for children and teenagers over the course of the past 20 years. And every word she writes and speaks makes you realise just how deeply she cares for children and how well she understands how they feel when, for example, they are forced to be quasi-parents to a bipolar mother, as in The Illustrated Mum, or they have to deal with motherlessness and the death of a beloved pet as does the child in The Cat Mummy. Patron of the children's cancer charity Momentum, Wilson works hard to help sick children, too.

Then, last year, heart trouble pulled her up short, putting her in hospital. And although she has made a good recovery, it has forced her to rethink her work schedules.

"I used to swim 50 lengths every morning but now I get up and lie on my lovely Victorian chaise longue and write for an hour, longhand. I find being still sleepy is a good time for creativity. Then, later in the day, I aim for a couple of hours to put that day's work on the computer and revise it."

The swimming is now occasional, partly because Wilson doesn't care for being accosted by excited children in the changing-room. "I like children very much – but only when I'm dressed," she says wryly. The adults with whom she used to swim before breakfast were more circumspect, providing the day-to-day chat missed by people who work from home, as Wilson has always done.

"I try to avoid hypochondria, but I do have to be careful these days," she says, telling me ruefully that when she fainted recently in a hot, stuffy room at a reception, the organisers called an ambulance. "I expect they were thinking 'Oh God, she's died on us!'"

Being "careful" doesn't seem to stop her from taking on masses of commitments. I am fascinated by a chunky little leather-bound book on the coffee table. A well-thumbed prayer book? Surely not. No, it is Wilson's diary. "It's ridiculous," she says, picking it up and leafing through it. Every page is thickly covered with busy black writing. No wonder – as she admits – it is sometimes difficult to ring-fence writing time.

But somehow she does. Her meaty, new novel, Hetty Feather, is set in the late 19th century: unusual territory for Wilson, although I surprise her by reminding her of two of my favourite Wilson short stories, both historical: "Call Me Blessed" (1986), about the Virgin Mary, and "The Daughter" (2000), about a Hampton Court kitchen maid at the time when the future Queen Elizabeth is born.

There is a strong Wilson-esque girl as the narrator at the heart of Hetty Feather and it is as much of a page-turner as anything she has written. Hetty, a foundling, is brought up by very decent foster parents until the age of six, when she has to go back to the Foundling Hospital in London that "owns" her – a terrible experience for any child, although foundlings were better looked after physically than most poor children in the community.

"I am a Foundling Fellow," Wilson explains, "and this book is my response to the honour." Three inaugural Foundling Fellows were appointed in 2007 by the Foundling Foundation, with Duffield Clore funding and a brief to develop original, creative initiatives for children.

Wilson researched most of Hetty Feather from her own collection of 15,000 books in her spacious Kingston house, purchased five years ago when her library outgrew her previous, very small house. "Hetty Feather was a perfect convalescence project," she says, explaining that a life-long love affair with late-Victorian Britain meant that a lot of the book's background was already in her head.

She is now three-quarters of the way through a new book – probably to publish in March 2010 – about a troubled child with celebrity parents.

I have seen dramatisations of several Wilson books – such as The Lottie Project and Secrets at the Polka Theatre in Wimbledon. Isn't it very difficult to hand over your "baby"? "No: Vicky Ireland, my usual adapter, is a novelist's dream," she says. "She won't change storylines because it upsets children in the audience who know the books but, skilled actor and director that she is, she finds wonderful ways of presenting it on stage." Wilson has also been generally happy with TV adaptations such as last year's Dustbin Baby (BBC).

I ask her about critics, since I remember the MP Ann Widdecombe vitriolically declaring that the subject matter of The Illustrated Mum was unsuitable for a children's book because the two girls belonging to the mother of the title have different fathers. "I try to reflect real life," she says. "I'm not condoning any of the sad things in my books, just acknowledging that they happen and that children have to deal with them." Then she grins: "After all, when dear old Phyllis [PD] James writes about murder, no one assumes she approves of it!"

As a girl herself, Jacqueline Aitken, who grew up in Kingston upon Thames, didn't shine at school, and left at 16. Then she moved to Scotland to work on a girls' magazine. There she met and married a printer named William Wilson and moved back south with him.

Her only daughter, Emma – now a Cambridge academic – was born when Wilson was 21. "I was a young mother lucky enough to have it both ways," she says. "I could look after Emma full-time during the day and write at night. That made us very close and we still have one very special holiday together each year." Her marriage lasted for more than 30 years before ending in divorce.

Justly proud of her achievements – though neither of us gets around to mentioning the DBE, the first ever given to a children's writer, in the 2008 New Year Honours – Wilson is keen to tell me about her ongoing work as Professorial Fellow at the Roehampton Institute, part of Kingston University. "I teach six seminars a year to a selected group of MA students on the Children's Literature and Creative Writing courses as well as giving an open lecture," she says, adding with a grin: "Not bad for someone who left school at 16, eh?" It's hard to disagree.

'Jacqueline Wilson's Tracy Beaker: The Game' will be available on Nintendo DS and PC from 11 September. 'Hetty Feather', is published on 1 October

The extract

Hetty Feather, By Jacqueline Wilson (Doubleday £12.99)

'...They opened a door at the end. It was empty apart from an old blanket and a chamber pot. "No!" I cried. "No, please – you can't put me in there!"

"Oh yes we can, Hetty Feather. You stay there and pray to be a better girl," said Matron... The door slammed shut and I heard the sound of a key turning. I was locked in! I heard their footsteps retreating. Perhaps they were simply trying to frighten me. They would come back any minute. They couldn't leave me locked in here'