Maria Shriver has written an excellent book to explain Alzheimer's disease to young children. I was helped and encouraged by her simple words and I'm an adult! I would highly recommend it for the children of families struggling through the difficulties of an aging family member who suffers from Alzheimer's.
This book is written in a way that would help children understand what happens with dementia, alzeheimer's, aging etc. I found it helpful for me too, because it's caring and positive, but not "childlish". I recently learned that my brother had Demential with Lewes Bodies. It would also help parents to deal with these issues when trying to explain to young people in their family.