I used the quiet and serious voice on a kid I was babysitting once. My stepmonster had volunteered me to babysit; I would never have agreed to watch this kid on my own free will. The family attended the church and the mom had something to do with her older son, so she dropped off the toddler at our house for me to watch.
I had watched over this kid in the nursery during Sunday School so I already knew he was a screamer and a temper tantrum thrower. Completely spoiled rotten. Things were going pretty well until he asked me for a drink of water. I said yes, in a minute, and took more than 0.03 seconds to move toward the kitchen to get it.
So the kid threw himself down on the dining room floor and proceeded to scream and wail as if I’d just punched him in the face. (My stepmonster was ignoring me and the kid, oblivious to all this.) I do not like this kid at all, so I did not hesitate. I picked up the little dude by his arms and set him on his feet, and stayed down on my knees so I was looking him straight in the eye. I took my hands off him and got right up in his face, so our noses were maybe two inches apart.
In a very low, quiet, but very menacing voice, I said, “Stop that right this instant. Maybe screaming works in your house, but in this house, we don’t act like that. If you want a drink, you may ask for a drink. I will get you one. Do not scream in this house. Ever. Got it?”
Eyes big as saucers, the kid nodded his agreement to not be a bratty little shit for the rest of the day and we got along just fine. He even said please and thank you for the water.
In this instance, I think we had a case of harried, crazy-busy mom who doesn’t necessarily have enough time to give the kid the kind of quality attention he needed. So he’d throw down a tantrum to at least get some attention, even if it was negative. I straight up let him know that he’d get plenty of positive attention from me (I attentively played with him all day) but I wasn’t having screeching, screaming toddler tantrums stomping up and down on my last nerve.
When the mom showed up, she was shocked to find her kid quietly playing with our collection of babysitting Legos. IIRC, we were building a racetrack or a rocket ship or something and he didn’t even notice she’d shown up to pick him up.
I think there’s sometimes an appropriate time and place to put your hands on someone else’s kid. All I did was pick him up and put him on his feet and then went to The Voice.