Hey, I just noticed your thread - I hope you've set to email you a notification or you come back to check sometime.
Congratulations on booking! You are going to have an absolutely awesome time!
I've been looking up St. Martin and St. Kitts myself. Now, keep in mind that I have not been to either place yet, and what I'm about to say is based solely on research, not on experience.
St. Martin: There is SO MUCH to do in St. Martin! The hardest part is figuring out what to do.
For us, the winning excursion is the SeaWorld Explorer, which is a semi-sub. A semi-sub is a vessel that is partially above water and partially submerged, and the submerged portion of the vessel has windows or portholes that let you see the sealife around you. The SeaWorld people send a diver out to feed the fish in front of your windows, ensuring you see lots of fish and maybe some turtles. You can book directly from the provider for $39 in advance, plus a $1 "mandatory donation" to the local marine wildlife park (who won't let you drive through to get TO the sub if you don't pay up) at the time of arrival. Booking direct will get you 1 hour 15 minutes in the semi-sub. By contrast, booking through your ship may get you a shorter period of time in the sub. The company representative said that the average taxi fare was $18 for 2 people (and taxis may charge more for additional people). If you book through your ship, you get transport included; however, given how much more of your day is left before and/or after, you may be better off paying for taxis so you don't have to return to the ship and then head out again.
Another option is the Butterfly Farm, which is $12 at the door if you just show up (no reservation needed), or is surely included as a stop in several ship-sponsored tours.
There are some old-fashioned sailing ships - the Captain Morgan and the Lord Sheffield. In particular, the Lord Sheffield looks interesting for kids because it has a bit of a pirate theme going.
As for St. Kitts, I have not yet seen anything at all that looks like it is geared for children. Maybe the scenic railway ride might work.