Ok, here are what the excursion books list that all ages can go on. These are from dave's site, they may not be the most recent, but close.
ST. Thomas:
-St. Thomas Beach day-beach with a lunch buffet. It is a Sapphire Beach. I'm not sure about adequate snorkeling.
-St. Thomas Island tour-a tour on a bus with little kids, I don't think is what you're looking for!!
-Coral World & tour- Coral World (aquarium & beach), but I believe you have a very limited amount of time at Coral World to swim and see the aquarium.
-Paradise Tram- Tram ride up the hillside for scenic views.
St. Martin:
Under 2 flags island tour- again a tour in a bus!!!
French Riviera Beach- I know nothing of the snorkeling here.
Butterfly Tour-I've heard this is pretty good, but no beach and no snorkeling
See & Sea Island tour- land tour and a ride in a semi-submerged sub (there are windows to see the fish through)
Afternoon beach bash-This is at orient beach. It is a 30 min ride. 2 hours at the beach. I don't know how the snorkeling is.
There you have it for
DCL excursions.
There are more available if you are going to split up groups. There are many that will take 5 years and up.
Good Luck!! Trying to choose, especially with that many is very difficult!!!