We did your exact cruise last August, between hurricanes.
In Grand Cayman we did the off-shore "swim-with-the-stingrays" excr\ursion, booking with a private (non RCI) company. It was the highlight of all three stops for us. (By the way, or kids are older teens.)
Not much to recomment in Jamaica. We used the same private company that had a tour of the Island, it was very nice. The Falls are OK, I guess you should do there. You can just jump over by a short cab ride.
At Laberdee, just relax. Walk around the Island to the "other side". Your kids might like the banana boat ride; ours did. You DON"T have to make arrangements in advance, just walk over after you get settled on the Island and get a slot.
If you have anyother questions, feel free to e-mail.