This is a kid who has all the stereotypically adorable childlike mannerisms: thumb-sucking, lisping, puppy-dog eyes, and so on. Really, though, they're just faking it all; they've figured out it's a good way to get what they want. Sometimes characters like this are Really Seven Hundred Years Old (and probably took the form of a small child precisely because it lends itself to being manipulative). On the other hand, they might just be kids who've figured out their elders' emotional cheat codes. In Japan, this often overlaps with the burikko, that is, a fake Kawaiiko.