I have only cruised once before with kids, and found that tour groups with a toddler or baby are not super fun, so I am planning on doing things on our own.
At St. maarten, there is a beach right near the shopping. You take the water taxi ($6 pp for the entire day) and then you can do the beach and shop. Here is a link with some
Great Bay beach -
http://www.sint-maarten.net/St-Maarten-Beaches/GreatBay.html
If my kids are in a good mood, I am probably going to venture to the French side to this beach at Le Galion.
http://www.sint-maarten.net/St-Maarten-Beaches/LeGalion.html. We will take a taxi for our party rather than do the Disney beach (plus they go to Orient Beach which I am not interested in). I know that my DD (4 1/2) will not be able to snorkel or go in a boat at
castaway cay and the water may be too cold for the baby, so this might be the only chance they have to do those things. Plus, the other side of the beach has big waves and surfers which my 8 year old will love. If we do this, we will leave first thing in the morning after docking, spend a few hours on the beach, then head back to the ship for a late lunch and probably go shopping (without kids!).
At St. Thomas I think I am going to just go with hubby by ferry to the French side. It kind of depends upon DS(8) school work. He is in a program that requires lots of projects so depending upon where he stands for projects and homework, I may turn this day into an educational day and take him on the St. John excursion that tours the sugar plantation and he can do a project on the sugar trade. I am really hoping to just have a day with DH though