In St. Thomas there are just a lot of excursions to pick from, see what all your cruiseline is offering. We did the "wild thing" in St. Thomas once and it was a gas, nice snorkeling trip with loud music and fast boat, bet your group would enjoy that?
In St. Maarten you may want to try the America's Cup Regatta, where you actually learn to sail and do an actual race. Some of the jobs are a lot of work, while others are not, you get to decide what all you want to do, its also a lot of fun.
If you just like a nice beach day, there is Meagans Beach in St. Thomas, although it does get crowded, but you can take a cab there yourselves and just leave some shopping time. We like to take a cab to Red Hook and then the Ferry over to St. John/Trunk Bay for some really good snorkeling. This does take some time, but the island of St. John is picture postcard beautiful, so something else to think about.
a lot also depends upon what the kids like to do. If they want to laze on the beach and do some snorkeling then you should head over to St. John's Island - Trunk Bay when you dock in St. Thomas, you might also consider taking the SkyTram to the top of St. Thomas for a breathtaking view.