I would coach the timing, because an overlap is only useful if it changes the picture.
If the full-back goes too early, the defender can track the run before the winger has even fixed anyone with the ball. You can also end up with two attackers in the same wide lane and nobody protecting behind them.
The reference I’d give is: “Can the player on the ball actually use your run?”
Sometimes the best support is staying behind or inside until the winger has controlled the ball and engaged the defender.
A 2v2 or 3v3 wide-channel game is useful because the full-back gets repeated chances to decide whether to overlap, underlap or stay.
At U12, I would not stop their willingness to join attacks.
I’d just help them see that a good attacking run is about timing and relationship, not simply running past the winger every time.
