Amy, I'm very sorry about your mom's diagnosed. I have a nine year old daughter as well and when my husband was diagnosed (going on two years now) we were so adamant in the beginning that she not know this would end in death. That as things progressed, talks with her would progress. Telling her he would eventually die didn't seem relevant, because he could live for years and we didn't know how to explain this to her without scaring her. We even went so far as to not give her the name for the disease, we just explained it as a neuromuscular disease.
Long story short, she started seeing a psychologist about a year ago for anxiety. It was of significance to this doctor that we hadn't told Sara fully about the disease and that her dad would die. The more she has been with this psychologist, the more I understand how extremely important it is to be up front and honest with children, no matter how hard it is. Sara picks up so much from us, our moods, our stress, dad's frustrations, and she was making up her own conclusions about what would happen to her dad. She even thought he was going to get better because he was in his wheelchair more and resting his legs. She would play with her dolls that dad was going to the doctor and getting better. It was also really hard on her that her dad couldn't do things with her that he used to and she just didn't understand why, she thought it was her. It was a rough first year or so and if I knew then what I know now... things could have been much different.
Anyway, I just wanted to share our situation. The ALSA sent us a great kid's packet full of information and games to help them understand this process, your local chapter may have something as well. I've read thru the link Al provided too and it's great. I think how you tell them also depends on how your child absorbs information. Sara only wants a little at a time. She will ask a question, get an answer and she's done for a while...
Sorry so long, but I hope this helps a little.
Ashley