The problem is that not all children are always supervised as they should be. One day when my son was only about 2 yrs old, I took him to the park that has a sandbox area. He had just started walking, and was still a little wobbly. He was sort of in the general vicinity of a sandcastle that had been made, but wasn't that close. All of a sudden a boy that appeared to be about 8-10 yrs old just yelled as loud as he could "Don't step on my sand castle!! !" My son was practically a baby then, and this big kid was yelling at him. My son was balling his eyes out. I said nothing to the little boy because I was in shock, but looking around, I NEVER saw anyone say a thing to this kid. I think this kid probably had some issues, so where were his parents or supervisor???? There is NO reason for an 8 or 10 yr old kid to yell at a 2 yr. old.......
So what I am saying is that I am seeing it a lot these days that kids are left unsupervised alot...
I think your best bet is to talk to the teachers or whoever is in charge so that they can keep an eye on it, but I still would not feel bad about reprimanding the kid.