It's easy! With a family of 5, I would definately rent a car and do things on my own. Taxis on St Thomas and St John charge per person, I think it would be cheaper for you to do it with a rental car and you'll get to see much more of St John.
Here's a secret for you, when you go to St John, travel past Trunk Bay and you'll find very nice beaches that you can have all to yourself!