On St marten, we did the Butterfly Farm/island tour. We went by bus, stopping to take pictures at an overlook to the open market in Marigot where we shopped for a little bit, finding tropical shirts for my DH and DD for tropical dinner. Then we went to the butterfly farm which my DD (age 9) loved--we had never been to one before. I wish that we had been able to spend more time at the farm, but it was an enjoyable excursion, and not too pricey.
On St. Thomas, DD and I grabbed a taxi to Coki beach and Coral World. I think the kids in your group would love this aquarium, then we went to the beach which had very gentle waves, white sand and tons of little fish swimming around. The taxi was $7 per person each way (with a group like yours you can probably negotiate this lower); I paid $23 for Coral World for one adult and one child (look for coupons online, or in tourist magazine to reduce this cost), admission allowed us to come and go to use their bathrooms, and fresh showers. I rented one lounge chair for $5.