Hello,
I am so sorry that this is happenig to you. My husband died 10 years ago when my daughter was 18 months old. I think that often it is harder for the adults than it is for the children. For my daughter, it was harder for her as she got older and other children would say things like "You can't play with us, you haven't got a daddy"
I would echo what another post said, check what the other child knows first. And then there are books that you can read to your child
"I'll always love you" is a story about a little boy whose dog is dying, and the boy is very sad but knows that the dog knew how much he meant to him because he had told him. You could make a good bye card/help your child write a letter to your ill friend (you don't have to give it to him, but the act of doing it may help your child).
At my husband's funeral, my daughter's nursery had helped her make a goodbye card and it was part of the ceremony that she put this card on the coffin. It helped her understand that it was goodbye.
One of the things the staff at the hospice told me at the time is that it's important not to tell your child that he's going to heaven without further simple explanation as this is too abstract for smaller children to understand. What's the difference between going to heaven and going to the shops? It's just another place...
There's another book called "Waterbugs and dragonflies" which talks about what happens to the waterbugs when they climb up the lily stalk and never come back. I found that really useful with my child.
There's another book called "Lifetimes" which explains death very simply and matter of factly i.e Trees live for 100 years or more, then they die. That's the way it is, that's their lifetime. It goes through a lot of different species and then gets to people, explaining the difference between just getting a splinter, and sometimes someone's body just can't carry on anymore, and we die, although usually that when happens when we are old.
I explained to my daughter that there was something in her daddy's body that wasn't supposed to be there (the cancer) and that made his body wear out just like clothes and shoes wear out. And that she would never see him again. It didn't mean that she or I were going to die because hopefully our bodies wouldn't wear out until we were very old, and it was rare to happen to someone then they were young.
There's a website called The Child Bereavement Network and there's a link on there called "are you a young person whose friend or relative has died?" There's also places like Winston's Wish which can give advice.
There's also memory boxes, memory jars where you can store the memory of the person with your child so that they are prepared and can talk about it before hand. Most of all, be honest..
I hope that this helps. Sorry that the post is long I'm not too sure on Mumsnet etiquette as it's my first time posting here - but it's something that I care about deeply, and I do tend to be verbose anyway