For what it's worth, if you want to try Boma, you don't have to worry about your picky teenage niece - there will be plenty for her to eat there. Boma has lots of African-inspired food at the buffets, yes, but there's also straight-up American stuff as well. Salads, soup, carved meats, chicken fingers, mac and cheese, bread, fruit, desserts - stuff like that. Plenty for everyone, every palate.
That said, if you don't want to do Boma/a buffet in general.... Beaches & Cream is a good place for a burger and sundae. You could head over to Disney Springs and eat there (pick your favorite location). Or you could stay a little longer at Animal Kingdom and have dinner at Yak & Yeti (Chinese food) - I've never done dinner there, but we've been surprised by how much we enjoyed eating lunch there previously, and Americanized Chinese food is pretty nonthreatening stuff for a picky teen.
Do you have park hoppers? If so, you could pop over to a different park for dinner and a nighttime show...