Well, a kiss on the cheek is basically the only kind of sexual harassment that a six-year old knows about (usually, hopefully). It's wrong. Yes, about as wrong as punching someone. Leaving a bruise is not the sole definition of physical harassment. Violating someone else's personal space, not just smack talking and "getting up in their face" but physically grabbing them and doing something is something that should be nipped in the bud.
Now, for a first offense, what's appropriate? Usually just a reprimand. For a second and third, what's appropriate? Well, it depends on whether or not verbal reprimands work. Counseling? Grounding? Having to go talk with a principal? Suspension? Suspension is probably wildly inappropriate for a first offense, presuming that someone just didn't know better.
Sometimes, though, the thing a person is punished for is the straw that broke the camel's back. Maybe they were dropping ice cubes down people's shirts, maybe they were "sycing" people out and making the person flinch or smacking them on the shoulder or back "playfully" hard enough to cause pain and finally, finally, verbal reprimands every day all week for just being a mean kid in the school classroom just haven't seemed to do anything and the thing that ostensibly they're being "harshly" punished for is just the straw that broke the camel's back. I've seen that a few times before.