How young are they? The bay islands are pretty much all about diving. But if you go to Roatan, which I would recommend for kids over Utila, and stay at a resort they would probably have courses for kids. I would probably imagine it would be set up so that they could do some sort of intro to SCUBA while the parents are out diving. If they are too young to dive, less than 8, I would wait a few years. The only things to do besides dive are to take island tours, maybe go to one of the few beaches, and a few other things. They can always snorkel though. And snorkeling is great and can be done from the shore in most places.
Oh and there have been a lot of whale sharks here the past few days. I have gotten to swim with them 5 times in the last 2 days.