In Cozumel there is a dolphin swim offered at Chankanaab National Park. There were young children enjoying this excursion. Some parents were standing on the platform watching. Children under 8 must come with a paying adult as a companion. Check out www.dolphindiscovery.com
for all of your questions. It addresses Cozumel on this site.
Belize offers great water activities if your family is into that. There is also the Baboon Santuary, the Zoo, cave tubing, Barrier Reef snorkeling and/or diving, swimming and snorkeling at Goff's Cay. Gail's point is a great place for spending time with sea manatees in their natural environment.