Two years ago we sailed with a 3 yr old. We did the Land/Sea tour on St Martin and a beach excursion on St. Thomas. She loved the Land/Sea tour, it included a little underwater sub and a bus ride around the island with a stop at the market area. If you don't want to be on a bus a long time I wouldn't suggest this tour, but we did like it. We got rained out on the beach in St Thomas. So that was a waste of time and money. The sun came out as we were leaving to head back to the ship.
Last year we sailed again on the Eastern and felt more confident that we could go off on our own. On St Martin we disembarked and went to a beach within walking distance. We had a great time and came and went as we pleased. On St Thomas, we walked to a tram that went up the side of the mountain. It was so cool! At the top is a restaurant and shops and an exotic bird show. My DD loved the exotic bird display and show, she even was chosen to be helper! We have photos of my family posed with exotic birds on our heads!! There also are hiking trails with labels on trees and plants.
Anyway, my point is...it was easy and cheaper to go off on our own. We had just as much fun and my daughter loved it. This Jan we are sailing on the Western. My DD will be 5. We plan on doing our own thing again.
Whatever you do, have a wonderful time!!