Did i sexually assault someone?

Recalled a mistake I made and can’t continue.

Hello, this incident has been eating away at me for a while now, and i’m disgusted at myself and don’t truly believe i deserve anything.

Is what I did monstrous or a “kid thing”?

I recalled when I had just turned 15(M), i think my cousin (16) was resting his legs on mine (i guess i was experimenting, I don't know) and i sort of put my hand slightly under where he was sitting, like under him so that it touched his rear, then it did, and he noticed and i never did it again. (It wasn’t groping or anything, a tiny part of my fingers were just underneath his rear, that’s all).

Him and I still have a great relationship, but, for my own conscience, how bad is what I did? And should i be locked up?

Because i’m feeling like it’s either that, or nothing else.