Ofcourse there is a Heter for everything. In this case when the Rebbe who has allowed himself to smack the kid, should be smacked right back by any of the parents. Ideally it will be twice as strong, some rabbonim say one should multiply by the weight of the child being hit. example if the child weighs
60 pounds, and the rebbe 240 pounds you can hit him 4 times. And if he asks why, tell him you got a Heter from a Chashuva rov. And he asks which rov, tell him the same one that gave you a Heter to hit my child. Also tell him its in the Gemmara. or that Rashi allows one to hit to end a dispute.
Hatzlocha raba