You can get up close to the dolphins at Seaworld and you can buy food for them and literally hand feed them.
Timing is key to this though as they open the food shack at certain times and the queue can be quite long.
Mine will be just 11 and 8.5 this year when we go.
First year we did all the Disney parks, Universal, Seaworld, Blizzard Beach and Typhoon Lagoon (which are waterparks).
Last year we did Universal, Islands of Adventure - twice, Wet n Wild (did not like), Seaworld, went to Clearwater for the weekend (has beach) and went out on a boat looking for dolphins which was successful.
This year we are doing Discovery Cove, Busch Gardens, Aquatica, Cape Canavaril. We may go back to Universal only because the boys want to do the new Simpsons ride if it is open.
With regards to Discovery Cove - although you get a full day at this park, you only get half an hour with the dolphins. There is only you can decide really if it is worth it or not.