I love MM and have been three or four times over the past couple of years. There's nothing in the show that I would worry about, as far as kids go. There are a couple of raunchy songs, but raunchy according to that time period, not modern standards.(One of them is called "Maids When You're Young" if you want to get an idea of what I'm talking about) There may be some jokes as well(The script changes regularly, so I don't know the exact story line you will here), but they will likely be fairly mild. Most of the shows and humor involve the Duke getting into trouble with the Duchess in one way or another. I've always enjoyed the food and ale there as well.
As to whether your kids will like it, I think it depends more on the individual children than the show itself. Most of the show is dialogue, so if you don't think he would be able to follow along and enjoy the storyline, that part might not be good for him. However, there is always portion of the show in which two knights have a live duel(With real weapons and audience members judging) that I think he would definitely enjoy. It is more of a play/dinner theater setting for the most part. Considering the deal with the Groupon though(Which I've used as well), it would probably be worthwhile to at least try it out if you think he might like it.
The shows generally run about two hours, I think, although it may run a little after 9:00pm at times. The beer is very good as well