My first impulse would actually be protection - I would run over, place myself between my child and the adult so they couldn’t touch them again, and ask them what the fuck was going on - it may of course be that they ‘deserved’ it (we’re not smackers, but I know other people wouldn’t hold back).
But not violence, I’d rather get my kid out of there and away before I thought about attacking the other person.
Slap my child, I slap two of yours.
Good thing I have no child.
My instinct is to get away and call the police.
I’m not a violent person. I’m not sure I’ve ever taken the first swing, especially as an adult.
If an adult laid a hand on my kid? I’d probably be taken away in handcuffs shortly.
My kid got hit by a flying toy at the park not terribly long ago and I went mental*, and that was an accident most likely by another child, so a grown-ass adult hitting would just bring out a side of me that I didn’t think existed before I popped out a booger factory.
*Grabbing the object and looking around for a guilty face. I think I yelled a few times “Whose is this?” I kept it. You hit my kid, I keep your toy.
Not a parent, and I’ve never hit anyone in my life. But if someone assaulted my kid, I’d be mighty tempted to respond with violence. Hell, even if someone assaulted one of my cats.
I’m sure you meant to say that you’d grab your baby, THEN get away… Yes?
Interestingly, there’s a TV series/book that presents a very similar situation. And in this case, said little kid really was begging for a smack.
I went with the low option but a lot would depend on how he reacted when I confronted him. Is he somehow handicapped? Does he seem under some kind of altered state? Is there a cop handy? Does he apologize or offer any kind of explanation? If the answers to all of the above and a few more are “no” then I would clock him.
(Back when my cousin was living with us we had her at a park and a lady (about 20-25) started pegging rocks at/near her. Not serious enough to cause damage but not acceptable behavior either. Turned out the lady was maybe IQ 85 and in her mind she was playing. I just asked her to stop and she did and the universe was fine.)
I would carefully and calmly beat him mercilessly.
Something like this would seem appropriate.
In my mind’s eye, the assailant has my child by one arm, although I realize that wasn’t specified in the OP. (Also, he’s male, which is probably dickish of me, but it’s true.) In which case my impulse is to run towards them, grab my child’s arm and back away with my child, other arm reached out in a “keep (the fuck) away” gesture. When we were several feet away, I’d probably ask, “What is going on?”, with my arm still outstretched.
Y’know, like Mom used to fling her arm across your chest like it was some sort of super seat belt, instead of a great way to cause additional bodily harm in the event of an accident. Mom Arm. It’s got powers.
My gut reaction is to think there was probably a good reason for such odd behavior, but I want my kid by me before I hear him out.
Violence is high on my list, but the first thing I do is assess the situation and the stranger. No sense getting myself killed over this.
What happens next depends upon the conclusion I reach in step 1. Violence is by no means off the table.
Or, y’know, “he’s.” Which would justify the comma in the sentence. 
For me, I’d be furious, but I’m really really not good at violence. I’m not sure how quickly that impulse would occur to me. It’s likelier to occur to me later on, once the complete shock of the situation is over.
Not a parent, no maternal instincts to speak of, at least none that apply to creatures with fewer than four feet.
My first reaction would be the same as it would be if anyone hit any person I cared about: Charging over, breathing fire, and bellowing WHAT THE FUCK ARE YOU DOING? in a tone I actually call the Everybody’s Mother Voice. It’s the same one you use to stop your toddler from wandering out into the street for the fourth time in the past half-hour, as you’re sprinting into range, right before you scoop him up and remove him bodily from the situation.
I expect that if the hitting continued after that, I would start in with violence. I can’t remember anyone ever not stopping whatever they were doing dead when I use the Everybody’s Mother Voice, though, even if they weren’t the one I was yelling at. I am informed I am frightening when angry. I am a small woman, and like many small women, I have also learned to fight extremely dirty, when called for. You freakishly tall people, you never remember to guard your knees. Or your nuts.
I don’t think I would actually commit manslaughter. The point of the violence would be to physically stop the assault long enough for both the target and me to get away. Although if I did knock him to the ground in the process, I expect I would sit on the fucker and bend a couple of his arms the wrong way to keep him there while I told his victim to call the cops.
Is this a trick question?
Assuming that this is an open and shut, slap, my kid crying, could only possibly the one adult, well then I only know of one way to make sure there isn’t a second violent assault on my child and that is to neutralize the perp so he isn’t a threat. Whatever that takes.
My kids are now bigger than me and would probably whomp the person in question, but when they were four I would have almost certainly beat the person in question like a government mule. If the dog we had when they were that age had been around, someone would have had teeth in their butt.
When my six year old daughter was raped my first reaction was to call the police. I had no desire to hurt anyone, even someone who had just raped my child. I was in shock and the rapists were pre-teen boys, but they were rapists nonetheless.
So many times I’ve heard people claim if someone hurt their child they’d never make it to the jail. They have no clue what they’d really be like if their child was hurt, but in my experience the need to help the victim comes before the need to take revenge in her name.
I would totally be capable of violence if someone assaulted one of my cats. I would be in jail. No doubt about it. Nobody lays a hand on my cats.
With that firmly in mind, I’m sure I would be even more capable of violence if someone assaulted my hypothetical child–as in, was causing the kid actual harm. A slap–I’m not sure. I’d be pretty pissed but I’d want more details.
Not a parent but I think my first instinct would be to go to my child and see if they are hurt.
Revenge would have to wait.
I was at a park once where this little boy was throwing rocks at the other children. Several adults told him to stop but he didnt. Finally a man blew up and started yelling at the kid (no he didnt touch him). His grandmother, who was sitting a ways away reading a newspaper and not paying attention, came running over to defend her kid.
Basically every parent just grabbed their kids and left.
Worse part was the little boy was black and all the other kids were white and the grandmother turned this into a racial incident and accused the man of picking on the boy because he was black.