When I was 13 years old I did something very wrong. Back in the day in my middle school was fucked up trend: slaping girl's asses. I didn't have the courage to do it myself so when we had family reunion I decided to practice with my 5 years old cousin. When I was playing with her I slightly spanked her butt over the clothes multiple times. Then I continued with slightly pulling her pants down and then I slightly spanked. Once I had pulled so much that I saw her private parts and in that moment I had realized that I messed up. Since then I have never repeated such disgusting behaviour, yet my guilt and selfhate for this action has been slowly increasing.
Nowdays with my cousin I have good relationship, I sometimes help her out with school work. She doesn't seem to be uncomfortable around me.
Am I a disgusting person?
I hate to break this to you, but kids are idiots. Their brains are still baking, so they do dumb shit all the time.
There's a reason that kids have a separate prong of the justice system in many places. Everybody knows the sheer stupidity that can happen because they, too, have done abysmally stupid shit.
Now, this one was pretty fucking bad. But it's still just an idiot kid being an idiot, not some predictor of the adult the kid grows into remaining an idiot.
Mind you, plenty of adults are idiots too, but the ones that don't repeat a given form of stupidity are ahead of the rest. So you're fine. Let go of the guilt, stop beating yourself up, and move on.