With little ones, in Jamaica I would consider the Rose Hall Beach Tour, plenty of sand and sun with lots to do. You could make a run to Margaritaville by taxi. If you dock in Montego Bay, the ride to the falls is long.
Grand Cayman---Stingray City...you've gotta do it at least once. Then go back to the village for some shopping and don't forget the rum cake.
Cozumel---everything everyone has suggested would be great for your kids. We did some shopping here too!
My baby is 18, so we will be zip lining in Jamaica with a trip to Margaritaville. Scuba in Grand Cayman (and buy rum cake). Dolphin Encounter (the one where you swim with the dolphins) or scuba in Cozumel, with a run to the beach, Carlos and Charlies and I have to pick up some vanilla in Cozumel.