The winner of the Glen Dimplex award for her compelling first novel about a terminally ill child, Sally Nicholls tells Fiona McCannwhy she was not afraid to cover territory that children's books usually avoid

1. My name is Sam. 2. I am eleven years old. 3. I collect stories and fantastic facts. 4. I have leukaemia. 5. By the time you read this, I will probably be dead.

THIS IS THE first of Sam McQueen's series of lists, compiled to help him grapple with the questions the grown-ups in his life won't answer, such as why God makes kids ill, and why people have to die. Not the kind of questions always associated with children, perhaps, but they were the inspiration for Sally Nicholls's astounding new children's book, Ways to Live Forever, which follows Sam's "scientific" attempts to make sense of his own mortality.

"I had a friend whose mum died, and I remember being amazed that she'd just sort of vanished off the edge of the world, and I started thinking: 'Well, where's she gone?'" explains Nicholls with disarming frankness. "We live in this very bright and vivid world, and we just pop out of nowhere, and then we just pop into nowhere again, and nobody knows what's happening."

It's a dark conversational topic for breakfast, particularly on the morning after a ceremony in the Irish Writers' Centre at which Nicholls was declared the overall winner of the Glen Dimplex New Writers Awards. Yet this north Yorkshire-born writer is matter-of-fact about the kinds of things more often spoken of in hushed tones or in adult codes over the heads of those who make up her readership.

"I wanted to write a book that asked these questions," she says. "I'm not a religious guru. I didn't want to answer the questions, but I wanted to say they're worth thinking about."

Though aged only 25, Nicholls, while sweetly diffident when her Glen Dimplex win was announced, reveals a dogged confidence when it comes to her work. In the course of her research for the book, she came up against disapproval about her subject matter and her character's persistent questioning, but Nicholls, whose soft-spoken shyness is underpinned by a steely purpose, would not be dissuaded.

"I thought, well, I'm the only person who knows Sam, my character, and I'm the only person who knows how he would react and how his family would react in that situation," she says. "So as long I get his character right, if I say he's going to ask questions, he's going to ask questions."

THE RESULT IS Sam McQueen, a heartbreakingly convincing 11-year-old eager to understand his own impending death.

"The emotional stuff is not something I can get advice on. That's something I know," says Nicholls - and her instincts have clearly paid off. Not only has Ways to Live Foreverbeen hoovering up awards - it also won this year's Waterstone's Children's Book Prize - but Nicholls has received countless e-mails from parents of ill children who have commended her treatment of the subject.

"It's a big relief," she admits. "It's actually something that has such a big emotional impact on real people. I just wanted to make sure that this was right."

What has clearly helped is Nicholls's evident understanding of children, and the lack of condescension she brings to her readers. "I write for me, aged 10," she says. She was 22 when she began writing Ways to Live Forever and feels the proximity of her own childhood helped her find her voice.

It's a childhood she describes as happy and secure, despite her father's sudden death when she was just two years old. Although she is quick to point out that she did not grow up in a "house of mourning", she is aware that his death had an impact on how she viewed the world as a child. "I think it taught me that powerfully unexpected things just can happen to ordinary people," she says.

Sam, his parents and his delightfully stroppy little sister, Ella, are just such ordinary people.

"They're not extraordinary characters," says Nicholls emphatically. "They're living quite a small life, but at the same time they have this huge emotional story."

In telling this story, Nicholls doesn't shy away from the kind of responsibilities that come with writing for a young audience, and is aware of the power stories can have over children.

"You tell an adult that we've got global warming and the world's going to melt, and they're like, 'Well, I've lived here for 40 years and the polar ice caps haven't melted yet' or they think, 'There's nothing I can do about it so I'm going to carry on living my life as I am'. They're very good at setting issues in the context of this huge amount of lived experience," Nicholls says. "But you tell a child that the ice caps are going to melt, and they either have nightmares or they ask if they should buy a boat because they'll need a ride to school. Every piece of information you give them is much bigger in their mental landscape."

Yet Nicholls makes no attempt to preach any didactic or religious message, offering instead what she sees as a variety of explanations and examinations of death and mortality, and allowing her readers to draw their own conclusions.

"I wanted to give children a range of different stories, and to say to them: 'Look, here are the things that some very intelligent people have thought might happen to you after you die.' And then give them the space to make up their own minds," she says.

GIVEN HER OWN considered, respectful approach to her readership, it is little wonder that she rails against celebrities who choose to write children's books for the wrong reasons.

"Maybe I'm doing a disservice to celebrities, but you do feel that some of them are thinking: 'Oh, I can't write an adult book, that's too hard. I'll write a children's book! That's easy.' "

It's a misapprehension that clearly irks her. "Actually, that's not the case. A children's book is as hard, or harder, to write than an adult book."

Nicholls makes it look easy, however, having already completed a second book, Season of Secrets, since the publication of Ways to Live Forever, with a third also on the way. Though thoughtful and serious for much of our interview over porridge and tea, a childish glee emerges when the subject of her writing career comes up, offering a glimpse of that 10-year-old self for whom she writes.

"When you're 10, you think: 'I'm going to be an astronaut when I grow up.' And when you get to 25, you realise that actually to be an astronaut isn't open to you any more. I just feel so grateful that when I was 10 I wanted to be writer. That was my 'being an astronaut', and I get to do it, and I get to do it all day, and I get to write stories, and people like them, and I just find that really, really amazing," she says.

She takes a breath and adds, with the kind of easy positivity evinced so endearingly in Sam McQueen: "I'd be quite happy if I got to be a writer for the rest of my life, really."

Ways to Live Forever, by Sally Nicholls, is published by Scholastic Children's Books, paperback £4.99