We've taken kids a few times,
Be sure to roll out something for them (and any high-metabolism adults) to eat every couple-few hours - just travelling on a boat burns up a lot of energy moving about and keeping your balance. Hungry (and they may not realize it) equals grumpy.

The beach in front of Pirates in the Bight at Norman Island is perfect for kids to play in while you relax in beach chairs, shaded by palm trees with a bar/restaurant 20' away. Also, we were there yesterday and the snorkeling was better than the Indians and the Caves, which we hit on the way in.