If you're familiar with the Harry Potter books - and let's face it, if you're a parent, then you are - you will know that there are only two characters who don't have a problem using the name of Harry's arch-enemy, Lord Voldemort: Harry himself and the wise headteacher of Hogwarts, Professor Dumbledore. Everyone else shudders at the mention and prefers "You Know Who". But Dumbedore knows that this is merely to add mystery to Voldemort's evil ways and so increase his power.

You might say that the word "cancer" works the same way in the muggle world. People don't like using it: it's the "big C" or the "C word" - You Know What rather than You Know Who. Even medics avoid it: there's no such thing as a cancer doctor; you go to see your "oncologist". But if such avoidance strategies occur when adults speak to one another, how on earth are we supposed to talk to children about a serious illness such as breast cancer?

Despite its reputation as a disease which chiefly affects post-menopausal women, breast cancer is the leading killer of younger women. That is, women who often have young families. The death rate is coming down, but it still kills 13,000 women every year (and some 35,000 women develop it every year). To put it mildly, a diagnosis of breast cancer is not good news.

When, in 1996, my late wife called me, from the hospital, in tears, with that news, I felt a vast, vertiginous chasm open up in front of me. I also felt terrible that I wasn't there with her, although the reason I wasn't was because we had been lulled into a false sense of security about her "benign" lump. But in that moment, all our plans, all present stability, all confidence in the future, were shaken to ruins.

Our twin children were little more than a year old. The issue of what to tell them seemed secondary; there were so many questions about diagnosis, treatment, prognosis. But for children of all ages, the questions of what to say, how much, and when, are critical.

"It's a very difficult and emotional time, and parents often don't know how to talk to their kids," says Liz Reed, clinical services manager at the charity Breast Cancer Care.

But what is the right thing to say? How far should you protect children from the scary truth that their mother has a life-threatening disease?

David and Vanessa Serota's daughter and son were six and four respectively when Vanessa was diagnosed with breast cancer 10 years ago. Within weeks, she had a lumpectomy.

"Our attitude was one of openness, rather than 'nothing is wrong'," says David. "We were up-front that 'Mum is going in for a serious operation.'"

Of course, it can be hard to explain technically both what cancer is and what treatment does. Serota relates that his son, the younger of the two, asked if he could see his mother's "plaster" when visiting after the surgery. Expecting a plaster, "He was more than impressed," he recalls.

What determined the Serotas to deal frankly with their children was a previous - and very different - family experience of cancer. "Vanessa's mother died of ovarian cancer," explains David. "Being from an earlier generation, she kept her suffering away from her kids. Rather than carry the additional stress of hiding the illness, we decided to be open."

"Holding information can be a burden," agrees Reed. "Many people reported being frightened about being open, but then felt a tremendous sense of relief when they sat down with their children and talked."

It is in response to such feedback that Breast Cancer Care has just published a booklet on talking with your children about breast cancer. Besides advice on how and what to tell children for each age group, the booklet contains fascinating snippets of parents' experience. "My daughter was only 10 months old when I was diagnosed," relates one, "but we realised how much she'd taken in about the treatment later on when she started giving chemotherapy to her dolls!"

Because cancer treatment can be pretty brutal, the instinct to protect children is powerful. But children pick up more than we give them credit for - especially moods of anxiety and depression. Of course, you don't want to alarm children, and you want to stay positive, but being realistic and explicit are also important.

"Children are very 'concrete' thinkers," we were advised by a counsellor. They can be almost shockingly pragmatic: a typical response to news about a parent's illness is, "So who will take me to school?" If, initially, you bridle at the apparent self-interest, you soon learn to be grateful that children seem so programmed to ensure their own physical and emotional wellbeing; they just need the information.

Decoded, of course, what they're really asking for is reassurance about continuity of love and care. But very often children don't give you much choice about "protecting" them. As Reed says: "Children will ask the dreadful questions that adults daren't."

The lesson I learned is that it's the adults who need euphemisms; children find the unknown far more frightening than the truth. It really is a matter of "better the devil you know" than You Know Who.

· For further details, see www.breastcancercare.org.uk.