My son has loved MM every time we went, and we started when he was 4. But then again, he's a dare devil and really tall. So at 4 he was already able to go on many of the coasters. Plus they do have a big kid's area for smaller kids.
I think it really depends on the children. If they are over 48 inches, then there are plenty of rides for them to enjoy. But if they don't like coasters, then there's not much.