Sounds like you're going on the Inspiration. We're going the end of this month with our kids. They're older than yours so some of the things we're doing probably wouldn't work for you. In Cozumel we're going to Chankanaab park for swimming and snorkeling. I haven't been there before but I have heard from others that have that it is very kid friendly. In Grand Caymen we're taking Soto's Cruises to the sting ray bar. It's $24/pp. Our excursion in Belize is cave tubing and the minimum age for our tour is 10. Haven't planned anything for Costa Maya. Probably just going to hang out on the beach.