When we were taking kids, snorkeling just past Saba Rock had some cannons strewn about that were cool. Yours are old enough to kayak and stand up paddle board. Hike up behind BEYC, there's a trail
Same at Norman Is. "Maroon" them on Sandy Spit for an hour. Last time we were there snorkeling off the beach at Cooper Is we saw a lot of fish and an octopus.

Be advised they'll be burning lots of energy and need to eat more often than the adults, especially an afternoon snack like mac n cheese, when you hit the beach bars in the afternoon just order some food for them with your first round of painkillers and when it eventually shows up put it in front of them, keeps away the grumpies.