Located in Fantasyland at the Magic Kingdom, Ariel's Grotto offers children and their parents (aka big children) a chance to meet and take a picture with Disney's most famous mermaid. There is also a small play area that features plenty of chances for children to get wet before or after they meet Ariel. Ariel will sign autographs and take photos with any and all comers, so this is a perfect place to get a family photo thanks to the beautiful backdrop that's brightly decorated with shells, starfish, and plenty of Ariel's treasures.
Based on The Unofficial Guide: The Color Companion to Walt Disney World, the best time to visit Ariel's Grotto is before 11 am and after 9 pm. Based on personal experience, I have to concur with the Guide.