The Frayed Extension Cord
I explained to my children that MS is caused by damage to nerves in the brain. I used the example of an extension cord. If an extension cord is frayed or cut, and the wire becomes damaged, electricity can't travel through the wires and the extension cord may not work properly.
In this example, the outer part of the extension cord represents the myelin sheath (protective coating) and the wires represent the nerves. I think this explanation is simple enough for even a child to comprehend.