We have two children, ages 10 and 7. We have decided not to do Sting Ray City because I think my youngest would be very scared. Having said that, we rented a jeep on our last cruise and my kids thought that was a good as the cruise! We are planning on going to the Turtle Farm, exploring some beaches, maybe do some off shore snorkling, maybe a horse back ride on the beach, and definitely a rum tasting (even if it is only their famous rum cake!).
Lisa A: I would get your car reserved prior to the cruise. I'll bet you can do it on-line. Remember, the steering wheel will be on the right side of the vehicle.
I think your children will enjoy the Turtle Farm. There are several spots along 7 mile beach where you can pull over and walk down a path to the beach. Be sure to take water sandals or water shoes, you'll need them.
AND I think your children would enjoy Stingray City. EVEN if they opted to stay on the boat just watching others interact with the rays in the water.