I was smacked as a child and it hasn't done me any harm.
My Dad used to take off his belt, and threaten me with it (never actually used it though), and the threat was enough to grt me to say sorry, or do what I had not done.
I was also caned at school several times. It was supposed to be a deterrent, but it obviously wasn't. lol
Anyway, I think it should be up to the parent to decide on whether a bit of a slap is deemed to be necessary or not (my sisters used to get smacked on the back of their legs - ouch!). When you see parents slapping their kids with a lot of force, and making them scream, is when it gets out of order in my opinion.