My own mama bear overreaction came before I was a mother and did not involve protecting any children. Gulp.
A few years ago I was going down an alpine slide. In case you're not familiar with those, it is a concrete track going down a mountain, with sloped sides to contain your sled, which is on wheels. There is a brake lever that allows you to control your speed. So you can go pretty fast but you have to brake a bit on turns. It is possible to go so fast you fly off the track. One of the primary posted rules is that there is no contact between sleds, and an attendant is at the top to ensure there is enough time between riders that one rider won't come too close to another.
Anyway, I'm sliding down and having a grand old time when all of a sudden I get SLAMMED from behind. It was so unexpected and such a jolt it was like my heart skipped and my hands got all sweaty. I look behind me and there is a kid, laughing his ass off. Probably around 10 years old. While I had been going pretty fast, this kid probably was just pushing the pedal to the metal, which is normal for a 10 year old. I then went the maximum speed so he wouldn't bump me again, but still slowed a tiny bit on a curve and that's when he SLAMMED me again. I think the fact that he was lighter reduced friction, so there maybe was no way I could have gone as fast as he could. He was laughing like a manic and I was scared and I was mad. I had felt like I could have completely lost control on the track especially given the timing of when he slammed into me (right on the top of a big curve). Breaking bones was an easy possibility.
What I did next came with no thought at all. I pulled up my brake entirely to come to a complete stop. As expected, he slammed into me again, laughing, but I was at least prepared that time. Now that he was stopped (he couldn't go unless I did) I turned around and told him, in a heart-stopping mama bear voice (not yelling but a lot more than firm - like fireballs shooting from my eyes intensity) that he was NOT to slam into me one more time. Fireballs, I'm telling you. Then I turned forward and descended the rest of the way without looking back. I actually thought I might have just screwed myself, because I figured (I was not a mother, remember) that a kid that reckless and intent on hurting others would not listen to me and probably go on a rampage and then I'd have to walk down the damn mountain carrying a 20 pound sled (or hope someone would call an ambulance if I needed one). Sigh. But it didn't happen, I just went all the way down, then looked up and he was not in sight. I got off and put my sled in the pile, met my husband and then I saw him coming down, going like 2 miles an hour, with a long line of a dozen people behind him, and him crying his face off.
So, yeah, wow, I felt this weird mixture of regret and annoyance and maybe even a little amusement. (Sorry).
I really did feel like he was being unsafe and reckless, and in retrospect I'm sure I could have told him with less intensity to stop it. But the problem was that the stakes were fairly high (I really don't think it was that unlikely that I could have broken bones if he caused me to lose control) and I did not know this kid so I had no idea where his "line" was. I didn't feel I had the luxury of increasing my intensity several times to see which point he would listen, and he had not demonstrated himself to be a careful or obedient kid at that point. So I do feel bad but I think my actions were defensible.
For the OP, I too still don't really understand why her first reaction wasn't to pluck her kid off the ladder when the other kid started pushing his shoe in, if she was right there. That would have been my move, and it wouldn't be because I am so much clearer-headed or anything, but it just seems like it would more immediately remove the safety risk. All I can think of was maybe she was right there but the ladder was so tall that the kids were out of reach. But I do know that when the stakes are high, we might overshoot a bit, and that's just how the cookie crumbles. I'd rather yell at a kid than have broken bones any day, you know? Maybe screaming was excessive, and clearly my eyeballs shooting fire went over the line with this kid, but with those experiences we can fine-tune our reactions. If I could do the slide thing over again, I might have still stopped my sled to get his attention so I could talk to him and also gauge his reaction, but said (minus fireballs) that it's not ok to slam, that I was losing control, and did I have his agreement to stop? I would have known for sure by his face whether we were cool or if I had a problem still, and if the latter, I would have just gotten off the track and let him pass rather than risking anything further. When we know better, we do better. (And I'm sure that lessons were learned by that 10 year old, plus the 7 year old girl in the boat, and so on).