How do you coach stopping a counter without coaching fouls?

Yes. You can coach stopping the counter without coaching cynical fouls. The first priorities are getting goal-side, controlling the attacker’s route and buying time for teammates to recover. If the defender can get close enough to use their body legally, force the attacker wider or make them slow down, the counter has already been damaged. I’d use 1v1 and 2v2 transition games where the defender scores a point for delaying the attack for four or five seconds, not only for winning the ball. That helps players understand that diving into a tackle is not the only way to defend an emergency. At U13, I would be very clear that grabbing, tripping or deliberately fouling is not the solution. The skill is recognising danger early and getting into a position where you can slow it without needing to foul.