Juliette Dann's daughter Aimee, four, was born weighing just 1lb 6oz and soon afterwards Juliette had a heart attack with near failure of her kidneys. Cystic fibrosis was diagnosed and she was told her only hope of living long enough to see her daughter grow up was a lung and heart transplant.

It was a tough prospect for Juliette herself, but she was more concerned for Aimee, feeling her daughter had already suffered enough. "I always felt after her birth that my body had let me down and therefore Aimee too, and being ill immediately afterwards meant I hadn't been able to care for her in the way I would have wanted," she says. "So it seemed very important to do everything possible to help her feel all right about my operation and I wasn't sure how best to do that."

She saw an advertisement asking for people who would like family therapy as a way of helping cope with an illness and who were willing to be filmed through the process for the BBC2 health series Talking Cure. She volunteered.

"I felt it would be useful to talk about the feelings I had about Aimee's birth and early life; that it might help me answer questions or spot anxiety.

"I was aware of a lot of underlying emotional stuff for the whole family and I thought it would be good if we were all part of the process."

So Juliette, her partner John - she is divorced from Aimee's father - and Aimee began a series of sessions at the Tavistock Clinic with Barbara Dale and Jenny Altschuler, specialists in helping families deal with life-threatening illnesses. Dale encouraged Aimee, as well as the adults, to talk about how she felt. Dale explains: "Parents naturally want to protect their children when they are telling them about a serious illness or operation, but the worries they have may not be so easily contained and the child picks them up. So the task is to help a child have the kind of understanding appropriate to their age. They need a coherent 'story', with a beginning, middle and an end, told in a way they can understand."

She goes on: "We work with parents to think about how, what and when to tell. In our experience children benefit from being helped to understand."

Obviously there are illnesses from which a parent might not recover. Dale and Altschuler, who work with many such families, think it important to help the parents decide how best the child can be told - and what they would like for the children after they are gone. Juliette feels positive about what went on for her in the therapy.

"I was concerned I might make Aimee into a mother figure while I was ill, but the therapy allowed me to establish with her that she was a normal five year old and she sailed through my operation."

Julie Stokes, a psychologist work ing at Winston's Wish, an organisation that specialises in working with bereaved children, shares the view that children benefit from honesty and can cope with complexity. She says: "If a mother or father is going into hospital for a major operation the children need to have that explained to them. They will know if a parent is being protective and will probably ask elsewhere, so we advise talking about the problem openly with the child - asking if they know what a hospital and an operation are and if possible showing them the hospital and ward where the parent will be. They should be told that the parent may be wired up and may look strange.

"Tell them you have taken the decision to have the operation because you believe it's for the best and acknowledge how the illness may have spoiled plans like going on holiday or life being as much fun as it was before."

Children may only want a small amount of knowledge at one time, Stokes adds. But if they do come back to the topic later, parents should be prepared to talk about it. "If there is a risk that the parent may not get better," says Stokes, "help them understand there is some risk, without making it too frightening."

Anna James went into hospital for an anal fissure, but just before she left hospital they told her they had found a cancerous tumour in her rectum and that she would need high-dose chemotherapy and radiotherapy. The memory is still painful. "I was shattered and I just assumed I was going to die," she says. "All I could think was what would happen to Charlie. There was a two week wait for the results of a CAT scan and blood tests. I tried hard not to show my distress. I had decided not to say anything until I got the results but he obviously picked up that things were wrong. He became cross and angry with me and that was difficult."

It was a relief when Anna learned the cancer hadn't got into her lymph glands and then she explained to Charlie. "He knew what cancer was so I knew he would be frightened. I wanted to say it was something else but my consultant said I must tell the truth because children spot lies and that can be worse than reality. I explained it quite clinically as a collection of cells changing in my body and that they were cancerous. I didn't want to be too melodramatic.

"I told him I had to go into hospital for treatment and that I might be tearful and bad tempered some of the time, because that was how the drugs affected people sometimes, but that it was the way I could get better - after that his behaviour changed. He was eating and sleeping well and it seemed to me he felt better."

Juliette Dann, who has suffered from mood swings and depression since her operation, was glad to have further therapy at that stage. She says: "There is an assumption everything will be all right after the operation, but in many ways that has been the hardest bit for me. I have been very glad of support and guidance in helping Aimee understand that everything hasn't immediately been happy ever after."

• Talking Cure will be shown on BBC2 tonight at 9.50pm.