Hi Elizabeth,
Thank you so much for visiting the Disney Parks Moms Panel with your question! I truly enjoy many of the tours at Walt Disney World. They add another unique layer to your magical vacation!
Disney's Dolphins in Depth looks like an amazing experience, with the opportunity to interact with dolphins at Epcot's The Living Seas Pavilion. Unfortunately, as detailed in the link that I provided, guests must be 13 years of age to participate in this experience. So perhaps your 9-year-old could take this experience if you head back to Walt Disney World again in the future.
I do have a good alternative for you. Consider the
Wild Africa Trek at Disney's Animal Kingdom. On this experience, you and a small group of other adventurers will go with guides for an exclusive tour of the savannas and other locations to see the animals from a truly unique perspective at Disney's Animal Kingdom. Along the way, you will get to traverse a rope bride, see the animals for longer periods of time, and even enjoy a delicious snack at a location on the savanna. This is truly an amazing experience. The minimum requirements for this excursion are that guests be 8 years old and 48 inches tall. Please see the website that I linked for further information. I took my daughter on the Wild Africa Trek when she was 8 years old and we had a wonderful time.
I hope that these thoughts are helpful for you, Elizabeth! Have a great time planning your magical vacation, and please do let us know if you have any further questions.