I was a Child Protective Services worker for several years and some cases of physical abuse on children were presented to me.. Usually they occurred because the parent lost control and went beyond mere "spanking" say and they left bruises and sometimes lacerations if a belt or coat hanger was used... Sometimes the child had to placed temporarily with a relative or if not available a shelter home... Usually attending parenting classes and supervised visits were part of the deal to return the child to the parents custody.
The problem as I see it is that if you say spanking is alright..to some that may mean using an object like a belt or a stick or a spoon .. and this inappropriate.
Some of the guidance suggested in the Baha'i Writings:
"Whensoever a mother seeth that her child hath done well, let her praise and applaud him and cheer his heart; and if the slightest undesirable trait should manifest itself, let her counsel the child and punish him, and use means based on reason, even a slight verbal chastisement should this be necessary. It is not, however, permissible to strike a child, or vilify him, for the child's character will be totally perverted if he be subjected to blows or verbal abuse."
~ Abdu'l-Baha, Selections from the Writings of Abdu'l-Baha, p. 125