How can i explain a child's muscular dystrophy to people?

Simple is best: Sometimes the simplest answer is the best, particularly when dealing with the curious or worried. The child's genes that are responsible for building and maintaining muscle don't work right. So the existing muscle remains weak & new muscle is not formed correctly.It is not contagious to others.

Briefly: First do not feel obligated to explain anything about your child to anyone. It really is not their business. If you want to explain then a brief factual synopsis is effective. Example: he has a genetic problem that leaves him with weak muscles...Or he has that problem that jerry lewis raises money for every year..Muscular dystrophy." something simple and to the point.