How much detail on defending crosses at U12?

I’d start with the defender’s relationship to the attacker before the cross arrives. If everyone is already facing only the ball, the runner has an advantage before the delivery. I want defenders adjusting their body so they can see enough of both the ball and the player they may need to deal with. Then keep the detail simple: nearest defender can pressure the cross, central defenders protect the most dangerous space, and weak-side players recover towards goal rather than staying wide. You can coach it in a 4v3 or 5v4 game with wide servers and lots of repeated deliveries. I would not give U12s a list of six box-defending rules. One or two strong pictures repeated often will transfer better: see ball and runner, protect the goal, and react to the second ball.