There should be a social worker at the hospital available for support. He/she will probably have a list of books to recommend for children. Also he/she should be able to give you an idea about what to expect as the disease progresses, so you can prepare your kids for visiting.
Also, contact your local hospice. They can be a really good source for information and support.
The 10th Good Thing about Barney by Judith Viorst is a good book introducing the idea of dying to children.
Mr. Rogers has an excellent small book for children about sickness & dying. It can be hard to find, but some libraries do carry it. I don't think it is available for purchase anymore.