The US Virgin Islands are a great spot to take the family on vacation.   No need to worry about a language barrier as everyone speaks English.
For families who love water sports the islands are a great choice.   There are many beaches with calm waters perfect for swimming or snorkeling.   Trunk Bay is a definite favorite on St. John.   When the cruise ships are in town this beach can get quite crowded.   One of the reasons it's a favorite is the underwater snorkel trail.   Underwater signs describing the local fish and coral mark the shallow waters.   It's a great guide for beginners and children on a first snorkel trip.  

Another fun treat for kids is to visit Paradise Point on St. Thomas.  Though it's also perfect for a romantic view of the city and sunsets, kids will love riding the 700 ft. tram to the top and getting a spectacular view of Charlotte Amalie and the island.

Coral World Ocean Park on St. Thomas is great.  If your kids love adventure they can touch live sharks as part of the animal attraction program at this hands-on aquarium.

If you shop around you can find hotels and resorts that offer specials for family travel. Some resorts will have kids' clubs or special activities for the young ones.  Ask on the TripAdvisor forums for advice on family-friendly hotels for the island you're planning to visit. They're a great resource.