What the law says
Parents have the right to make choices about reasonable punishments, but there is a fine line between "reasonable chastisement" (which is a defence in law) and assault (which is a criminal offence).
A smack might be considered to be reasonable chastisement if it is open-handed (not administered with a fist or any kind of implement) administered on a part of the body where it will not cause harm not severe enough to leave a mark.
Depending on the circumstances, an open-handed smack on a child's bottom, if it does not leave a mark, might therefore be considered "reasonable chastisement", whereas a smack on the head - even open-handed - might be considered assault, as it could cause harm.
It is against the law for teachers, nursery workers and childcare workers to smack another person's child. But anybody employed privately by a parent, such as a babysitter or nanny, may smack a child as long as the parent gives permission.