I found this fascinating quote today:
Children do notice behaviors. They are smart, and they often see more than we think they do. They also have active imaginations, and the truth may be less scary to them than what they have conjured up to explain behaviors they don’t understand. Your child may not come straight out and ask what’s wrong; it’s up to you to be sensitive to your child’s level of awareness.myelinrepair.org, When and How to Tell Your Child You Have MS, May 2010
You should read the whole article.