St Thomas is a shorter flight, so with kids this could be a great option. The Marriott in ST is great and you can always take a ferry over to St Johm, which I think has some of the most beautiful beaches in the world. The water is calm and warm. And the snorkeling is great. Lots of options to sail, snorkel, and relax. 

My family and I also love Costa Rica. The beaches aren't nearly as scenic as the USVIs, but there are a lot of fun activiies and excursions for kids.