You have no right to strike another person's child.
Cindy's neighbor's kid runs into her yard and up to Cindy and slaps her.
Cindy wants to respond to the slap.
Cindy decides to take the kid back to the neighbor's house and tell the parents what the kid did.
Cindy decides to slap the kid back for slapping her first.
The parents start shouting at the kid who gets grounded for a month.
The kid's parents come running and shout at Cindy for slapping the kid.