Adding my POD. That was not the time for a measured and sensitive discussion on the how's and why's of their son's behavior
On a scout activity of abseiling down a small cliff front, one family brought along their younger son who thought pushing small stones off the edge was no big deal (the side the scouts and people were working from). The parents were standing by and also thought this was no big deal, so my DH stepped in as one of the leaders of the day and told him to stop.
The boy continued knocking the stones off the edge, and my dh went off on him - his parents then chimed in to say he was alright let him be and my dh informed them it was not ok. Not for the kids going down the cliff and I certainly not for the people who were holding onto the kids guide ropes at the bottom, and if they couldn't understand that they could leave the activity. The parents were a little shocked to be presented with the option of comply or leave the activity, but they complied and led their youngest away from the cliff face to do something else.
See I think that's completely wrong. What should have happened was your dh ask the little boy to go to his parents and explain to them why it isn't okay for the boy to do that. They might have been completely clueless about where the rocks were going. For your dh to go off on him is not right.
Dangerous activity. Safety is imperative. The scout leader was absolutely correct in his role to make the offending behavior stop right then.
But there are ways to be immediate without "going off" on a kid.
Of course, we don't know what *actual, real-life actions/words/tone* the OP means when she uses the phrase "going off." To some people, that's extreme (I would never use that for anything but a really, really loud yelling, etc.).
And we don't know the age of the kid, and whether he's old enough to have learned to follow the directions of the leader.
But I would also say that just saying, "stop that" is not effective. Instead say,* "Stop that--those stones are hitting the kids on the ropes and the people down below holding the ropes. It hurts! and they can't concentrate. Someone will get hurt."
Sure it's longer, but it's also more CREDIBLE, and it doesn't leave the rock-kicker thinking you're just one of those fussy people who thinks children should stand still. (because they're out there--I've seen people tell kids to not kick rocks when they're kicking them further down the dusty country road)
(and again, we don't really know what the DH/scout leader said; we have a sketchy description by the OP)