In the beginning, there was “The Croods,” an agreeable enough 2013 cartoon about an overprotective early man named Grug (Nicolas Cage), who was so worried about the perils of the outside world that he kept his clan confined to a cave. Then his adventurous teenage daughter Eep (Emma Stone) met an outsider, Guy (Ryan Reynolds), and the family was forced to confront the possibility that maybe they had been a little limited in their thinking.

From that rudimentary concept, “The Croods” managed to conjure a surprisingly lavish CG world for these goofy cavepeople to explore — one in which the creatures and environments were so richly detailed, they easily distracted from the threadbare storytelling. (The movie was basically a feature-length sitcom treated as a runaway theme-park ride.) When all was said and done and the Crood family had learned its lesson, there was no organic reason to continue the story, but that didn’t stop DreamWorks Animation,...