If you go to Sea Grape Beach or Courtyard, just ask a taxi driver to bring you there - $4 per person each way, there are plenty of facilities. There is food, drink, rest rooms, outdoor showers, etc. You can rent all sorts of water sports, including jet skis, water skiing, parasailing, kayaks, etc. We loved 7 mile beach and had a great day. We were just adults, but it was very relaxing. There were lots of kids enjoying it as well.
love2travel - there is a turtle farm you could go to. There is HELL, which is a tourist trap type place with some rock formations. You will certainly want to look into Stingray City with your DD8 and if interested book with a private tour guide.