look into doing an excursion with the stingrays in Grand Caymen. Its a great excursion for the family,i would recommend using Captain Marvin.
I am not sure of the age restrictions for Dunns River Falls, the older one might be able to climb the falls with you. I would also look into spending some time at Margueritaville, fun family place or you could book a day pass at one of the inclusive hotels for beach access.
Cozumel - Channunkaab Park - great place for the family to spend the day. There are lots of family friendly beaches to choose from also.