I never suggested it was "okay" but seriously I think your reaction is beyond absurd. Calling the authorities over this IMO is a HUGE overreaction and a complete waste of the police departments resources.
It's his word against an adult's word. I'm only reading what the OP wrote and she's upset.
In the end, the adult is supposed to set the example.
My kids have gone to camp (not volunteer camp - one we pay dearly for) for years. They have gone on extreme wilderness trips, whitewater, etc. and NEVER has any staffman, guide, anyone ever had to "whack" someone to get them back on a trail.
If it was serious enough for my child to mention to me, it was serious enough.
If it had been meant as a joke or a way to get someone "back on the trail" my child would have explained it to me like that.
My children's (and other children's) well-being is worthy of the police department's resources. If it's a false alarm, so be it. But, if the scouts or whomever is sending out an adult who may be a loose cannon or can't control her group in an appropriate manner, then steps should be taken to either stop her or punish her if necessary.
And, if a child can't behave, his parents should, at their expense, have to come and get their child. My children's camp have even gone so far as to not allow a particular camper to go along on the trips if they can't follow the rules and behave in a safe and appropriate manner. They don't need to smack them around (on the tush or anywhere else) to get these kids to cooperate.
I am amazed that there are parents here that think it's okay for other adults to touch their kids in this manner.
I have to get back to work - I came on here to ask a question and this thread has me all side-tracked now, I forgot why I came here...