Kids U12 should not study openings at all, unless they are strong enough in the middle game and endgame. If they are that talented they should play main lines, but learn them by studying games of the Greats.
Assuming that these kids are relative beginners they should only know the three principles: development, centre and castling. Further they should be allowed to try out every decent move. Concerning their study - focus on tactics and practical endgames. That's what they are good at. Only at 12 or older (don't take this barrier too strict) they should touch other chess subjects.