I'm pretty sure St. Maarten has the most awesome beach I've ever been to. White cookie dough sand and crystal clear water at 2-3 different beaches we stopped at. And no one trying to sell us anything. St. Thomas is nice - but I liked St. Maarten better. Roatan also has some beautiful beaches, but they also have a lot more folks walking the beach trying to sell you something. We also really enjoyed the beach at Cozumel.

I'll second the recommendation for Royal Caribbean and one of their older Oasis class ships. They'll have kids groups, flowrider and water slides, arcade, etc. We took Harmony of the Seas and really enjoyed it.