Totally agree with IM point here. I think especially at a young age <10, children, like animals, respond negatively to pain. Think of how a dog learns what is too far and what is acceptable, the other puppies don't threaten to remove it's food, they bite it back - promptly. This then cements it into the dogs head that biting too hard (or being naughty in the case of children) is not good because it has consequences and causes pain. In 10 minutes the dog has forgotten all about the bite back, and is carrying on with daily life. When I was smacked on the hand for being naughty as a kid, after 10 minutes I'd stopped sniffling, and found something else to do. I'd forgotten all about it and was ready to move on.
I think when parent are too soft on children, it doesn't build up a resilience to pain. By the time I was 9 years old, the hand smacking had lost its effectiveness, and I wasn't bothered because it didn't hurt anymore. At this stage my parent would take things away from me, like pudding after meal times, sending us to bed early, when I was older they took my games console and didn't let me play out etc. When people's brains haven't fully developed they can usually only understand good Vs bad... Bringing the bad association of pain with the naughty act will cement this in the child's head and they will learn better than forcing them to think of doing something else to entertain themselves because you've confiscated their iPhone 24.
As long as it's only spanking, or smacking, so as to cause a red mark and nothing more, I think it's acceptable, when we are talking bruises and broken bones, clearly that is completely wrong.
My two cents!
OG