Resort recommendation for family of 4 @ Biscayne/Miami area

Kids are 10&12. We're flying into Ft. Lauderdale in mid May just for a few days and would like to hit some nice beaches to relax and also to snorkel at Key Biscayne. We also want to stay at a resort with big pools and slides. I prefer to stay in Key Biscayne but anywhere south of Ft.Lauderdale would be fine. I'd would really appreciate some suggestions.