As long as the corporal punishment given is not abusive, that is to say it is not severe or cause injury to a child, there is no reason it should violate international law. Although some may oppose all corporal punishment, there is no reason it should be illegal at a national or international level.
There is a fine line between corporal punishment and child abuse. Spanking a child isn't the same as starving a child to modify behavior. Spanking a child isn't the same as whipping your child to a bloody pulp. Parents should have the right to punish their children as they see fit as long as it doesn't break any local laws.