I performed cunnilingus as well as other things on my sister, who was 6. It was back when I was 8, we were in the bathtub together, and I asked her what her crotch looked like up close. She responded by sitting against the edge of the tub, spreading her legs, and waiting diligently. I quickly proceeded to prod and lick it for some reason I can't quite recall. She started moaning and shaking, and I asked if she was okay. She said she was okay, it just felt good.
I then told her to get out of the tub and lie on the floor. She did so, and I got out as well. I started sticking my fingers inside of her, asking if it still felt good. She barely managed to say yes through all of her moaning and gasping. I then asked if I could put my crotch on hers. She agreed, and I started to lie on top of her. Whilst I did, however, my genitalia slipped and slid into her vagina. I gave a slight moan of pleasure, asked her if she wanted to do it again, and she nodded her head. I then proceeded to thrust in and out of her rather sloppily, and after a bit, she moaned really loudly, locked her legs around me, and I started to cum. I told her that I was about to pee, but she didn't seem to release her hold at all.
A few seconds later, I cried out, thrust once more back in her, and came. We were both lying on the floor for a bit, recuperating, and she said it was the best thing she had ever felt. The same goes for me, and I can still remember it vividly.