If she is "obsessed" with dolphins--and you can afford it--TAKE HER TO SEA WORLD or DISCOVERY COVE!! Yes, I am yelling. If you really want to give her dolphins, you need to do Sea World or Discovery Cove. As much as I love Disney, we are all animal lovers, and Sea World is always the highlight of our trip. At Sea World, you can buy fish (about $1 a fish! So spend the extra $20 bucks or so if you can) and feed the dolphins. If you want to pet them, and interact with them, feeding them is the best way. They don't usually come up to you without a fish in your hands. (unless you are lucky enough to stand next to someone who is feeding) Have you heard of Discovery Cove?
www.discoverycove.com
You can swim with dolphins there, (it is owned by Seaworld and includes 7 day admission to seaworld) and if you really want to spend the money she can do a tour called Trainer for a Day where she can really get involved with the dophins. Think of it as an investment, maybe she will be a marine biologist one day, all because you made the time to explore her interest.
Trust me, if she is "obsessed" with dolphins, if you seriously want to do something nice for her take her to one of these places. It will be much better for her interests than extra time at Epcot or Animal Kingdom. Yes, there is a lot to see and do at WDW, but if she loves dolphins, take her to Seaworld/Discovery Cove and skip the waterparks or DTD. Truly, it's a no-brainer. See one or two dolphins in a tank at the Living Seas behind glass? or feed them, pet them, smell them, swim with them at Seaworld/Discover Cove...? If it was up to her, I know which one she would pick, LOL.
Of course, if you really can't swing it this trip, the Living Seas will be cool too! Maybe she can save some of her own money for the Seaworld/Discovery Cove option?
OK, I am pretty enthusiastic, can you tell we love dolphins too?

edited to add, my dolphin loving dd is only 6yo, and we haven't done the Discovery Cove dolphin swim yet, just fed and pet them at Sea World. I totally understand if it's a first trip (but not a once in a childhood trip) that you would want her to wait to really experience dolphins.