Nikkita - you have a couple of options. Dunns River Falls, one of you could climb the falls if you wished while the other one walks with the kiddos alongside the falls, lots of people do this because they want to see the beauty of the falls but the climb is a bit much for them. It doesn't take too long and the kids will be oohing, aahing and laughing at the people.
Mayan Ruins, definately a Not Do with little ones. The most family friendly thing to do in Cozumel would be to spend a couple of hours at Channunkaab National Park - nice clean beach,& facilities, you can do a little snorkeling, there is a replica of the ruins to explore, aquarium its a very family friendly place.