Slapping another adult on the bum without consent is assault or battery a lot of places. I'm in the UK where slapping your child is legal, but merely causing another adult to be in fear of imminent "unlawful physical contact" (an unwanted slap, for example) is assault even if it never actually happens.

The hypocrisy is astounding and one of the worst aspects - behavior we've outlawed against adults are being outright advocated for children in a position where they can't reasonably defend themselves.

You're seeking to justify violence and abuse, no matter how you trivialise it.