Thankfully it was usually during nap time, but sometimes it would be out in the open, because little kids truly don't give a fuck about what people think. This left me with the unfortunate task of having to explain to the kids to not do that at school (in a way that didn't scar them for the rest of their life or make them think masturbating was bad, which, holy hell, that's a lot of pressure). Then I had to bring it up with their parents when they came to pick them up. You can imagine how fun that was. One of the worst conversations I ever had with a parent involved me having to tell them that little Bobby tended to spend nap time jacking it when he thought no one was looking. Mom didn't take that well at all. And how could she? "You know your precious child? The little kid who loves pizza and marathons of Dinotrux? Well, here's the thing ..."
Oh, and mutual masturbation was a thing too. A horrifying, horrifying thing. I've ... I've seen terrible stuff. You don't know, MAN, you weren't there.