An expression I like that applies here is "people who say you shouldn't hit your kids, don't have kids." As an Eagle Scout, and frequent employee of the Boy Scouts of America, I have spent a lot of time working with kids, and I'll let you in on a secret I've learned, kids are shit heads.
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I'll even provide an example. During a "work day" in which the entire troop attends to perform monthly maintenance on the cabin that houses out organization. It's a big event and everybody pitches in. Well, in the cabin itself is a small space where people sign in and wait between jobs; if someone needed help, then they can go in there, and "enlist" whoever looks bored. Also in the room are coffee, snacks, donuts; breakfast food.
Well, during one faithful work day, I, acting as an adult went into the wait room to find someone to help out, I was pulling logs to a wood splitter and I needed an extra pair of hands. I found a kid sitting and doing nothing, so I went up to him and said, I'm paraphrasing here: "Hey kid I need some help moving logs." His response was, and I'm NOT paraphrasing here: "Go fuck yourself."
Needless to say I was taken aback, I found out his name, practically dragged him to his father, told his father what happened. And the kid starts lying through his teeth, telling his dad "he started shouting at me" and "I was going to help".
I have never wanted to kill a child before then. And that was a big tipping point for me on whether or not it's OK to hit your kids.
Of course a line needs to be drawn. If you smack a kid upside the head because he's being a little turd, that's one thing. If you hit your kids for no reason, or because you feel like shit, then you're a horrible person, and you deserve what your jailhouse roommate is going to do to you.