Welcome aboard, Laura! Nassau has some great dolphin experiences from which to choose.
If you're looking to explore Atlantis, by all means, check out the
Altantis Dolphin Cay Shallow Water Interaction and Discover Atlantis. You'll get about a half an hour in the water with these beautiful creatures, with more or less one-on-one time depending on how large of a group is booked on the same adventure. After this adventure, you get to explore Atlantis, however this does not include access to the beach.
If you're wanting to interact with dolphins and have a beach day, I'd highly recommend the
Blue Lagoon Island Dolphin Encounter. Not only will you get to interact with the dolphins in a small group, but you'll get to stay afterwards to enjoy the rest of the day with multiple return time options at Blue Lagoon Beach. My daughter and I absolutely loved our experience with the dolphins there a few months ago. Since we took the earliest departure, we had tons of time after swimming with the dolphins to relax and play on Blue Lagoon Beach. The staff at Dolphin Encounters was fabulous and the dolphins were wonderful. We felt like we had lots of personal interaction time with them and created memories we'll never forget! Though the maximum guest to dolphin ratio is 10:1, we lucked out with only 4 in our group and two dolphins, so my daughter and I had our own personal dolphin friend that day!
There were a few three year olds on the Dolphin Encounter when we were there, and everyone seemed to really enjoy themselves! We sure did!
I hope those details help as you begin planning, Laura! Stop back again with any other questions! I'm happy to help!