Pollo Compero at DTD has some wraps on the menu, including tuna. Cooke's of Dublin (QS by Raglan Road) has fish and chips and shrimp and chips on the kids' menu. Wolfgang Puck Express has salmon on the adult "big plate" menu, but no seafood on the kids' menu. And, of course, Earl of Sandwich has tuna salad sandwiches.
Captain Cook's at the Poly is good, and has a fish of the day on the kids' menu (that may be dinner only, but that sounds like it would work for you). And the Landscape of Flavors food court at the Art of Animation resort gets very good reviews, and has a little bit of everything. They definitely have a fish choice on the kids' menu.
Good luck!
Oh! A couple more: Sanaa out at Animal Kingdom Lodge - Kidani Village has a sustainable fish entrée on the kids' menu, and we love the restaurant. Also, if you really want to go all out, Flying Fish on the Boardwalk is fantastic. Pricey, but fantastic. They have baked fish sticks on the kids' menu, and if your DS is used to eating seafood, I imagine that they would pare down one of the adult offerings to kids' size. No promises on that, but they have exceptional service.