Short Answer: Your kid's bike is fine to hang by the wheels and yours probably is too, however; [insert long answer here]

Long Answer: Certain deep section carbon fiber wheels use a lightweight carbon fiber cone. The shape functions as a wing and doesn't support any load from the bike as it's only there for aero purposes. Easy test would be to just give them a squeeze with your fingers. If they bend in easily I wouldn't risk it, but if they're rigid then you can hang out however you please.