NIght, many years ago I worked for a TV production company and we made a documentary about a charity called Winston's Wish
They deal with exactly this sort of thing. Specifically (from the site):
Support for Serious Illness
Families face a tremendous challenge adjusting to the life-threatening illness of a close relative. This is equally true whether someone becomes ill or is disabled suddenly (for example, in an accident) or if they have a long-standing illness (for example, cancer). Families have to adapt to a different sort of life with all the uncertainties that this can bring.
If recovery seems unlikely, it can be unbelievably hard to accept that hope is fading and that a family member is going to die. It?s even more difficult to have to tell the children.
Where a death is expected, Winston's Wish can help children to understand the illness of a parent or sibling, and prepare for their death.
We will encourage communication within the family about the illness and individual thoughts, feelings and reactions about what is happening. We also work with the family to encourage the appropriate involvement of children and young people in the situation.
We can also support the person who is dying to prepare ?memory stores? for their children and to prepare letters and other messages to be opened on special occasions in the future (for example, an 18th birthday).
If you would like more information, or would like to talk about supporting a child when someone is seriously ill, please ring the Winston's Wish Family Line (0845 2030405).
We also have a book for 5 to 11-year-olds called The Secret C which explains cancer and a handbook for adults supporting a child facing the death or serious illness of someone close called As Big as It Gets. Both publications can be ordered on-line.
Sorry that this is happening to you