Stingray sandbar can be scary to kids because you have stingrays gliding around you and over you. I mean really close. Some kids don't have a problem but others can get upset. It really depends on how easily your kids get spun up by things they aren't used to. Our son loved it. He was 8 or 9 I think, when we first took him there.
For all it went through, Cozumel is looking remarkably good. Still lots of things you can't do, but still worth a visit. The San Gervasio ruins on Cozumel have never been fun to me - before or after hurricanes. If you are speaking of Tulum on the mainland, well, yes, it can be boring for little kids. It can be boring to adults too. There isn't much excitement to offer at venues such as the Mayan ruins. The kids would probably like Xel-Ha or Xcaret.
BTW - a personal perspective.......we have never left our son alone on the ship while we went ashore. I think it is a really bad idea with younger children. We don't even consider it. Too many bad scenarios.