Whether or not it's sexual for you does not change its sexual effects on the child you are beating.

You tap a child on the rear, that's a sexual area, and therefore sexual abuse. YOU cannot tell that child that it's not abuse when you are deliberately touching an area that is sensitive. I dare you to smack an adult's ass in public and tell them that it's not a sexual advance. The only difference is that the adult will likely hit back. A child is defenseless and powerless. It's a sick one-sided display of aggression that does nothing but instill fear, hostility, and resentment into the child, and opens them up to a life of sexual dysfunction.