A few weeks ago, at an event she was doing in Kingston upon Thames, south-west London, Jacqueline Wilson met "a dad who had brought his daughter from Liverpool. His daughter had become obsessed with my books, and so he'd stayed with a friend in London on the Saturday night, and then they caught the tube to Richmond and then laboriously they got the bus all the way to Kingston, and they'd gone to the theatre, they'd listened, and then they were in the queue for signing, and then he was pretty worried because they were going back on the coach from Victoria station so he did ask could they possibly come forward – and I just thought, 'What a fantastic dad!'" Her voice, caught in an almost child-like rhythm of "and then, and then", tilts so high it turns into a squeak.

Wilson, who has written almost 100 books, must, at this point, be beyond surprise at the lengths to which her fans will go to meet her, or, indeed, at how many of them there are: she has sold more than 30m books in the UK alone and, famously, once signed fans' copies for a solid seven and a half hours. But the unforced wonder with which she greets the phenomenon – not to mention the diligence with which she caters to it, staying for those seven and a half hours, for instance, and, until quite recently, maintaining a punishing schedule of school visits – is a large part of why the phenomenon exists at all. And although there are those who protest that her books are full of darkness — divorce (The Suitcase Kid), homelessness (Dustbin Baby), dead best friends (Vicky Angel), mental illness (The Illustrated Mum), children in care (The Story of Tracy Beaker, the book that launched her to stardom) — , anyone who's actually read them knows that an underlying belief in the resilience of children, an intact sense of wonder, an enjoyment of small mercies, somehow makes them reassuring after all.

Though it does help that all except one have happy endings – or, at least, what might be construed as happy, in the circumstances. Is that not a betrayal of sorts? Life tends not to work out like that – "I think when you're a teenager you can cope more with a more bleak ending." Her diction is slightly fey, but deliberate, every consonant given its proper place and due — the vocal equivalent of clear round handwriting. "But I think when you're young, books can haunt you a bit, and if you have a really sad and difficult ending that's a bit much. So I do try – it's quite difficult to get something realistic, and a book that will give children hope that you can come through horrible situations, that you can get to another place. And anyway, that's what I believe."

Hence all the letters, too: about 200 a week. Many, of course, are pure fan mail, full of the "sweet things that kids say". But a good number are also pleas to the one person who seems to understand – problems with a stepfather, for instance, after they've read The Suitcase Kid, about a child whose parents are divorcing. Wilson knows she has to be careful, writing back, because who knows who opens the post in that household, "so I say things like, 'I can see life's quite hard for you sometimes, but isn't it great that you've got a best friend' – some anodyne thing but that will hopefully show the child that I am on his or her side.'"

Questions from the audience at events throw up a different sort of challenge: how to answer a question from a child who is so nervous they haven't noticed the child before them asked exactly the same one; how to say gently, to a distressed 7-year-old, that she'll talk to them afterwards about their parents' breakup; how to deflect, increasingly (and to her genteel dismay), questions about money: how much she earns, what her rings are worth. For someone who is regularly pictured wearing at least one heavy silver ring on each finger, she is, when we meet at the Foundling Museum in central London, wearing disappointingly few rings: the long-fingered hands cradling a cup of cranberry, raspberry and elderflower tea are adorned with only a huge translucent grey stone, a crepuscular silver effort, and a thumb ring.Wilson has a preternatural memory for the details of her own childhood – how does she think childhood has changed? Less freedom, she says, immediately. "When I was at primary school, as long as you had a friend to go with you were free to wander and roam. I was walking to school by myself by the time I was seven." Yet at the same time, "children nowadays, really very young – because of television, because of changes in attitudes – they know so much more. When I was a little girl, even the word divorce was whispered. And I didn't hear a four-letter word spoken aloud until I was 17."

Is that a kind of loss? "I think children are very self-conscious now. They so want to be cool, and they want to wear the right clothes. Some children seem to have lost the ability to play imaginatively and to carry on playing imaginatively. I'm sure little kids still play with dolls and teddies – but now, most kids from seven up would not be seen dead with a doll in their arms. Or admit to their friends that this is what they like to do." She makes a point of telling children about her own dolls and soft toys, of which she has a collection in her home in Kingston (along with a trove of 15,000 books, a cat, and a good friend who cooks and gardens). "I don't necessarily play with them now, but they're very important to me."

Cultural knowingness doesn't account for all of it, however; parenting has changed. "When I was brought up in the 50s most children were on the periphery and mummy and particularly daddy's needs were put first." What effect does she think it has on the children? "I think it's lovely for anybody to be the centre of attention, and it's lovely that we're much more sensitive to children's needs, but occasionally" – she laughs, to take the sting out of it – "You think, 'OK! You've had your moment now, little so-and-so.'"

For all her native chirpiness, Wilson's childhood, as the only daughter of an only daughter who married the wrong man, does not sound particularly easy. Her mother was a bookkeeper who only found her métier as an antiques dealer late in life; her father, a draughtsman turned civil servant, was prone to frightening rages. "Biddy and Harry couldn't stand each other," she wrote in the first volume of her autobiography for kids, My Secret Diary, not putting too fine a point on it. Money was an issue, too, and until they got a council flat, when she was six, they sometimes lived with grandparents, sometimes in furnished rooms, Jacqueline's area designated by a rug in the corner of her parents' bedroom – "which can't have done much for the relationship either".

"I always said I wouldn't take sides, because that's so horrible. My dad was such a complex man. I could be very frightened of him. But on the other hand he would bring back books from the library that were really inspired choices. And he took me to the National Gallery, say, or for walks in the country, so he was tuned into all the things that I liked to do." Not that her mother didn't exercise her own hold – "it's interesting, she is 87 and only comes up to there on me" – Wilson, a small woman herself, indicates her chest – "but I must admit she has a way of saying my name that gives me a kind of frisson of fear."

And she can't hide the puzzlement that while her mother collects her books, she has never read one. "I would have thought, just out of simple curiosity, she'd want to see what they were like. But she says, 'They're children's books. Why would I want to read children's books?' And there is a logic in that. I can understand it – it's not a deliberate – I mean, my own daughter writes wonderful books, but she's an academic" – a reader in contemporary French literature and film at Cambridge University – "and certainly the first two or three were couched in almost impenetrable academic terms, quoting French philosophers and psychoanalysts, and I did struggle through, but the meaning remained pretty elusive." But her mother has "realised that I've become quite well known now, and to my horror, we're rather to the other extreme – anybody she happens to meet, be it the man reading the electricity metre, she quizzes them. 'Do you have daughters?'" Her voice becomes peremptory, an involuntary echo. "'Do they read? Have they read Jacqueline Wilson?'" And then she talks all too readily about my books, and I feel all that sort of turning embarrassment."

She left school at 16, which "was quite joyful", answered a DC Thomson ad looking for teenage writers for a new magazine – DC Thomson now publishes The Official Jacqueline Wilson Magazine, dedicated to her books and doings and merchandise – and headed up to Dundee to work for it, where a mix-up meant that she had to sleep in a linen cupboard, and, as she once put it, "I was so homesick I'd go into the Dundee Woolworths just because it smelt like the one near home!" When she met her now-ex husband (he left after 32 years of marriage), they moved south. She contemplates her early wedding with a certain incredulity. "I was nineteeeeeen!" Emma was born when she was 21, and although she'd always imagined herself with six children, "dressed in Laura Ashley, at the stove, with a toddler here and a baby there and little kids playing with dough" Emma was one of those infants "who for the first few months of her life didn't sleep more than two hours, ever, at a time. And my ex-husband made it pretty plain that if I wanted any more I'd be coping on my own. And I did still want some more, but never quite absolutely enough to make sure they arrived."

She is quite grateful now, because she has such a close friendship with her daughter – they go to galleries together, and travel – and "I never really had to make that awful decision that most women now have to make about writing – are you going to give up your job or carry on and have all the tearing your hair out about the nursery, the child-minder, whatever". In fact, for years Wilson not only always had a novel on the boil – she even made a foray into crime writing for a bit – but wrote at least 10,000 words a week for magazines, to supplement her husband's policeman's income. For years she swam 50 speedy lengths at 6.30 every morning and still averages at least two novels a year.

Wilson has the unmistakable air of someone who needs to keep working, hard, so as not to lose their luck, a feeling exacerbated by a diagnosis, two years ago, of heart failure. "The very words, heart failure, I thought, 'Bloody hell, this'll be curtains, immediately.' But thank goodness for modern drugs, and I've got a gadget in my chest – if I should fail utterly – it apparently has the kick of a mule. But, you know – you have absolutely no idea whether you've got an ordinary life span left. Common sense tells you you probably haven't."

She takes some comfort from the experience of a friend who was told he had two years left – a decade ago. "So there you go," she says, emphatic and slightly over-cheery. "You just never know your luck."