Feeding the dolphins at the Key West area. Each cup has 4 fish in it, and costs $4. They go very quickly. The dolphin comes up, stops in front of the person with the food, comes up out of the water, and waits a few seconds while the humans pet them under their nostrum. Then the kid throws the fish in their mouth and they swim away happy. Sure, the kid can stand next to other people on the wall and touch the dolphin, but they stop directly in front of the person with the food.
Bring lots of cash for this. I'd say at least 3 cups per kid. Good time to go is in the afternoon. At 3pm a trainer comes out with a bucket of fish for a few of the dolphin. Your kid can get some extra touches in if they stand next to this person, but it happens fast.