It's a stupid ass country then. It isn't abuse. It hurts certainly but hell even if you draw blood( and that is heavily over doing it and does in fact border on abuse) it wouldn't really do damage because of the way the ass is made. Lots of fat, no major blood vessels, or organs of any kind. I have in fact read multiple places where being shot in the ass is preferable to being shot anywhere else because almost all of the times an ass shot will just really hurt whereas even a shot in the shoulder has a reasonable chance of being lethal.
A smack to the ass with : hand, small implement like a leather belt, wooden paddle, things like that if used when deserved for anti-social and particularly disagreeable behavior when being told to stop fails to work. Is not child abuse. If anyone, be they: individual, organisation, or nation says otherwise they are both wrong, stupid, and contributing to the anti-social, hateful delinquency problem of much of the developed world's youth.
You can argue with me if you want but you are wasting your time as you will never convince me otherwise. Largely due to me seeing that children used to, on average, behave a great deal better decades ago than they do now. The main thing I noticed removed was physical punishment. Also it's parents not trying. You don't need to discipline your children if they behave to begin with or cease upon being told, doing so otherwise is abuse. However using it in proper cases seems to be one of the most reliable ways to both get a child to behave properly and respect authority.