I’d coach it as a shared defensive picture.
If one centre-back steps in with a dropping striker, somebody has to protect the space they have left. That might be the other centre-back sliding across, the outside defender narrowing, or a midfielder dropping depending on where the ball and other attackers are.
The mistake is when the stepping defender moves but everybody else keeps their original position.
I’d use a 4v4 or 5v5 directional game where one forward is encouraged to come short. Ask the defenders afterwards: “If he steps in there, what space has opened behind?”
At U12, I’d avoid giving one child a permanent covering job.
You want the group to recognise the chain reaction: one defender leaves the line, the others adjust. That relationship is much more useful than telling the centre-back never to follow a striker.
