Virgin Gorda has a ton to offer. There are ferries from St. Thomas and Tortola if you don't want to spend $ on flights. The Baths are beautiful and a great place for kids to snorkel and swim. There are diving trips that you can take from Virgin Gorda. North Sound has a lot of places to stay and you can go over to Prickly Pear for the day to hang out on a gorgeous white beach.
Another choice is Anegada but maybe a little harder to get to (although I do think there are ferries). The beaches are fabulous (the best in the Caribbean and I have been to almost every island) and the snorkeling is some of the best as well. I grew up in Grenada and we go back to the Caribbean every year during the Christmas Holidays. I have also been to Hawaii and I will tell you that the water in the Caribbean is like taking a bath compared to the water in Hawaii. The Caribbean Sea/Atlantic ocean is much warmer than the Pacific.