Definitely recommend the America's Cup Regatta. It is a great deal of fun and lasts about 2 hours... which will leave you plenty of time to do the beaches and/or shopping.
I also would recommend the beach on the left side of the ferry pier. Lots of watersports activities are available... including wave runners and water trampoline and slide. We rented wave runners for the afternoon and had a blast... and I am sure would be enjoyed by your kids!
Enjoy your cruise
BTW.... when in St thomas... one of the best tours is the tram ride to the highest point on the island. Gives a great view of the harbor in Charlotte Amaille and all of the cruise ships in port. It can be done at your leisure and is in walking distance from the pier