We did it in Chankanaab park in Cozumel so I am not sure if you are thinking of the same one. For some reason I am thinking that the minimum age is 8. Your niece may be happier with just an encounter. But if you do do the swim - we walked out onto a big pier and then down some stairs to an area where we then took a small ladder down onto a floating platform in which the water was about waist deep to an adult. Then after we were all standing there our dolphin came out and did a "show" and we petted him and kissed him for a photo op. Then we each swam out one at a time to the dolphin (a decent swim in very deep water) and the dolphin swam us back to the platform (ie gave us a ride). After everyone had a turn we then went for a freestyle snorkle/swim with the dolphin (the whole group). Then we went into the gift shop and spent a bunch of money on photos and a video!