Karen, The first time we went to MSM my son was 7 and my husband told him he could go exprlore - before we saw the beware of quicksand signs...then I was panicked.
We returned last summer and planned our trip so we would be there to see the tide come in. We all walked all the way around the mont - along with many other people. It is safe, but I would say it's probably a good idea to do it with someone, and as the other poster said - know when to expect the tide!
Here's a website that tells the tide times for the whole year. http://www.ot-montsaintmichel.com/horaires_juin08_gb.htm
Have fun, it's a really great place!