Of course there are worse things to do to children than smack them. Not that it isn't a pretty futile thing to do, and a very ineffective way of disciplining. Generally it's just a way of getting a message across to a child, and there are normally plenty of other ways to do that.

If it's hard enough to hurt so badly that a child is frightened into behaving in a different way, that's not "smacking", and it's not something that is tolerable. Anyone who seriously tries to shape a child's behaviour by means of pain is probably wasting their time, unless the pain is severe enough to do the job, which with a brave child could mean pretty severe. Severe enough to knock the courage out. Is that the idea?