Hi Joey, I recognize you from the LV forum :)
You can't go wrong with cabbing to Sapphire and spending the day there. It is a beautiful beach. There is a snorkel shack right on the beach where you can rent chairs, snorkel gear, floaties, etc. and they have drinks, etc. There is also Banana Cabana right there to get food and drinks and also beach waiters to take drink/food orders.
Or Megan's Bay is breathtakingly beautiful :) The water there is very, very calm. Not sure if your boys are good swimmers or not. If they aren't, Megan's may be a better choice and there are plenty of facilities there. It can get crowded on cruise ship days but if you go there just walk down to your left (as you are facing the water) and keep going till the crowd is thinner.
We've done the Paradise Point thing before and in our opinion it isn't worth the cost but to each their own.
We've stopped there on cruises several times and have also gone to stay. If I recall correctly on the cruises we were able to share vans to the beaches which brought the cost down. I'm thinking we paid about $6 each, each way last time. No worries about getting a ride back. You can either arrange with the driver who took you to come and take you back at a specific time or there are always cabs waiting around.
Enjoy St. Thomas! It is a beautiful island :)