Frankly, the government needs to stay out of child rearing, since they fail at everything they touch, and no government will care more about a child than the parent, nor does the government have to put up with an unruly, undisciplined child. That said...
Child abuse occurs when the discipline being applied would be severe enough to cause a lasting physical or emotional injury [NOTE: the occasional swat or spanking meted out properly will not cause either). If you break the skin, break a bone, or leave a scar, the discipline has gone too far. And even lesser disciplinary methods, such as spanking, can be considered child abuse IF the discipline is unwarranted or is used more often than necessary - spanking a child mercilessly every day "just in case he did something you have not found out about yet" is completely out of the question.
But let's look ar spanking, or swatting a child for discipline...
We can start with the Bible, which clearly states "Spare the rod, spoil the child." On this basis alone, the state should not interfere in spanking because it is a religious tenet. But that begs the question, WHY is it a religious tenet, and WHY do parents often spank a child as if it were an automatic disciplinary instinct?
Because it IS instinct. Man is an animal, literally. And if you have ever spent any time in nature (you know, the real world), you will have noticed that all animals use physical discipline to teach their young. A Mama bear will swat her cub so hard he will roll for 20 feet. A lioness will do the same. Even the mild-mannered rabbit will kick its young, to teach them when an action is wrong or dangerous. It is a survival mechanism. If our young do not fear the consequences of a certain act that could harm them, they will have less chance of survival.
There's an old saying (it got old because it's true) that you always hurt the one you love. That's because if you love them, you would rather hurt them a little in order to keep them from greater harm. If a spanking will teach a child not to play in the road, it could help keep him alive.
Of course, some bleeding heart, mindless liberals will try to tell you that you could accomplish the same thing by simply talking to the child. Hogwash! If that were true, then Dr. Spock's kid would likely not have ended up committing suicide. Dr. Spock espoused the theory that physical discipline is always wrong, and unnecessary. Apparently, there is something wrong with that theory if his own child was unable to cope with life.
Talking to a child as a method of "discipline" may make the parent feel better, but it does nothing to help the child become a strong, stable adult.
If you have to swat a child more than a few times in the child's life, you are either overdoing it, or the child is a demon that needs military (or reform) school. And if you never have a need to discipline your child, consider yourself blessed.