This reminds me of what we did when I was in 3rd grade.
I started I started "playing doctor" with the boy across the street. He was in 4th grade. We had a "fort" in his back yard that we'd fool around in.
I'm not sure exactly how it was suggested, but it started with peeing. Because of course we were having too much fun to go inside! It was a Sunday so I was still wearing a skirt from Church. The fort was like a kind of tent structure built against a chain fence. I pulled down my panties and lifted my skirt so he could see between my legs, and he pulled down his pants.
He stuck his wiener through one of the fence holes and peed into the wooded area back there, holding his shirt up with both hands. I remember it sticking straight out. Then I laid on my back and opened my legs in a split with my crotch against the fence holes and peed- it sprayed everywhere because I'd been holding it so long. I don't know why I did it in this position, I don't think I knew how to squat.
After that I kept my legs spread, and he fondled my privates, my lips and clit. He just touched randomly, wiggling his fingers around. I was touching his wiener which was pretty hard Eventually he laid on top of me and put his little dick up to my hole and slid it in and out. I guess we were just mimicking the only things we'd heard about sex.
I wanted to do it again, but I was terrified of getting caught, and I wasn't usually dressed for "easy access". There were other times we peed or I let him touch my privates, especially if I wore a long shirt to "cover up" or was in my bathing suit. This was way before the days for internet porn. I never told anyone this story, and I doubt he did either.