what if you just said something like:
"You know when Mommy has trouble walking or when she gets really super tired and has to nap? Well it is because my leg can't "hear" what my brain/head is telling it to do. When this happens Mommy has to use a cane/sit/take a nap till it talks right again. We call it MS for short - sort of like when we called those itchy spots you had chicken pox. Any questions?"

Answer in simple, short language - concetrating on what they asked and not going into too much technically detail. Children tend to be satisfied with simple, direct answers . They pop up with new questions or concerns over the coming months and years as they mature and can understand & relate to more.

My niece was 5 when we had to explain to her. Her biggest fears were:
1) are you going to die? Not until I am really old and you have your own little kids a long, long time from now.
2) does it hurt? Not really (ok, so I fibbed a bit)
3) can I catch it? Oh no - just like I couldn't catch your broken arm last summer
4) how come so & so said you will be in a home & dead soon? How silly - he just is scared and doesn't know what he is talking about. You ever want to check with me just say so.

I let little kids explore and play with my crutches and wheelchair for a few moments if they follow the rules {ask first, sometimes I need it and they cant use it and when I say "all done" then no complaining}. I think kids need to experiment and discover equipment to not be afraid of it and also to learn to be comfortable around disabilities, but that is my opinion.

Remeber that kids will ask questions thruout the months/years as they think of them and as they have the capacity/maturity to understand more. Keep it on their level and short & sweet.

good luck!