The National Cancer Institute www.cancer.gov and the American Cancer Society www.cancer.org have information on communicating with children. You can contact them directly for more information. Here is an example http://www.cancer.gov/cancertopics/coping/advancedcancer/page7
That's a difficult situation with an advanced cancer. I think honesty is the best way to go, and to let them know that they are going to continue to be taken care of no matter what. The love isn't going anywhere. At the same time that it's a horrible thing, it's also an opportunity that people who die suddenly do not have a chance to do-- to help them cope with these events before they are suddenly thrust onto them with an event, and to find help for them if they need it. My heart goes out to you and your family. When I was a young boy, my mom was very ill, and I was told she was going to die. I was probably 11 or 12. She ended up living after a period of illness. I think from that perspective, I wanted to know that I was going to be okay. I also would have liked something tangible-- a photograph and a letter or something-- to cope with the feeling that I was going to lose the memory of my mom. Of course you never forget, but that's what went through my head at the time. Many many years later, I can even remember the people sitting in the waiting room in the hospital. Emotions can sometimes be difficult, but they also have a way of burning memories into our brains. Give them something special to remember, and reach out to specialists who deal with this all day every day.