Lots of stuff to do here in Cozumel -- it's a very kid-friendly place. Because there are so many of you, you might even consider getting a tour guide for a half day which would at the same time serve as your transportation. You can design you own tour, which might include stops at several beach clubs as well as travelling to the other side of the island to check out the white sand beaches and open sea. You could grab a bite over there at one of the beach cabanas and the kids could play in the waves (swimming not recommended due to current). When friends come to visit and we don't have enough room in the car for everyone, we use Island Tours. Would be happy to hook you up it it sounds like what you want.
You can also go to cozumelinsider.com which has tons of information about things to do here on the island.
Ross in Cozumel