I agree that BlueZoo is the best, and it may have some seafood, but the emphasis is definitely on fish. For shellfish seafood, try Fulton's at Pleasure Island or the Crab House (two locations not far from Disney: http://www.crabhouseseafood.com/locat...)
Worth the drive for kids is RJ Gator's at Oviedo Marketplace mall. It has shellfish seafood and an "old Florida" theme with log booths that rock back and forth. Definitely a family place.
Probably the best seafood is at Todd English's bluezoo, on Disney property in the Swan-Dolphin hotel. Fancy and expensive but kid-friendly. For casual, the fried clams, etc., are great near Disney at Celebration Town Tavern in the town of Celebration. Flying Fish in Disney's BoardWalk Inn is good and family-friendly too, but fine-dining quality. That's a start.