My DD, 5, did it on the first Mexican cruise while we did the swim. They are in a smaller pool w/a very shallow platform that the kids are on. A momma dolphin and baby dolphin were in the pool w/them as well as one more dolphin (kids could not touch baby, he was just there to be w/his momma). DD learned about the dolphins, got to do the kiss (photo op for them) and also got to sit on the dolphin because of how small she is (smaller ones sat on dolphin's back, larger kids danced w/her). They were done in about half the time we were and then she was escorted over to our pool to watch the rest of our swim (she was not allowed in the water w/us at all...she and one other little girl her age had parents in the large pool...they sat and watched together but could not even put their feet into our pool as our trainers asked them not to do that for fear of them falling into the deeper water.
Overall, it was a really nice excursion for the kids...perfect for the 5 year old crowd!