A friend who recently cruised to St Thomas on another cruise line visited Coral World park. I found a web site by doing a google search for Coral World in St Thomas that tells you all about it and even includes a link to get a coupon for reduced admission. She said it is interesting, a manageable size, and right next door to a beach so you can spend part of your day swimming. It is evidently easy to get taxis between port and the park. We are planning on visiting on this cruise. There is a Disney excursion that includes Coral World but also includes another stop that I don't think would interest my kids much (the same age as yours), doesn't seem to really include beach time and is much more expensive than going alone, even considering taxi fare, so we're going to do it on our own.