Children should not be smacked at all. In families where there are other elders other than the child's parents it often happens that the child sees contradicting patters of behavior in them and tries to imitate it. This is when elders who do not like such behavior are inclined to punish the child. If elders take care to expose the child only to desirable behavior, then I think the child will learn only good things.
I know a family where the mother in law uses abusive language for the daughter in law right in front of her grandson who is only one and half year of age. This child unwittingly repeats the same abusive words to anybody at his own choice. The person who hears it may smack the child at no fault of his.