When I was 14, I babysat for an 11 year old boy every Saturday night when his parents went out to their card club. I think they mostly played Bridge. The boy really was old enough to stay by himself, I thought, but his parents didn't want to leave him alone. He was a cute kid, and I enjoyed the time with him.
The normal routine was for us to watch TV until about 11pm, with him laying on the couch and me sitting in a chair. The chair was positioned kind of behind the couch and to the side, so I couldn't see him unless he was sitting up. But what he didn't realize was that I could see his reflection in the glass door in front of him. They had a silvery film on it so it reflected well when it was dark outside. The chair I sat in was on the dark side of the room, so he couldn't see me.
One night I noticed he had his hand in his PJs. He always wore these skimpy shortie PJs pants (no shirt) that I enjoyed seeing him in. He had cute legs, and when he walked I could see his little penis bouncing, since he didn't wear underwear under them.
Anyway, I was fascinated by seeing him with his hand in his PJs, and I could see movement, so I knew he was playing with himself. For a while he lay on his stomach and humped the couch cushion, like I used to do when I masturbated with my pillow. Then he turned back so he was on his back, unsnapped his PJs, and lowered them slightly. I could see him playing with his little boner, pulling his foreskin back and forth. As he got more into it, I got really turned on, and pulled my own penis out of the legband of the shorts I had on, and stroked it gently, watching him the whole time.
He got going in earnest, and I could see him stiffen and hold still, with just slight motions of his hand, and I could see his penis jerk a little. At that, I lost it, and started ejaculating all over my leg and the arm of the chair. Luckily I had a paper towel handy and quickly cleaned up. While I was doing that he was putting his PJs back into position, and lay there relaxing.
He did that every time I sat for him, and of course I watched carefully every time and did myself. I especially remember a year later when he shot semen for the first time that I saw. I believe it was his first ever, since he looked really surprised and scurried to wipe up as best he could. He didn't have anything with him to do that, so he wiped it up with his fingers and stuck them between the couch cushions to clean them off. Some got onto his PJs, and when he got up to go to bed, there was a big wet stain on one side. I pretended not to notice, and when he went to bed, I went and felt and sniffed the fresh semen between the couch cushions, and jacked off again.
The next time I came over he had prepared himself with a paper towel, and shot into that, stuffing it under the cushion until he went to bed, when he sneaked it into the trash can in the kitchen. He didn't notice it, but I retrieved it and took it home, and saved it as a suvenier which I used while masturbating, thinking of him doing it.
He was getting old enough that his parents felt OK leaving him alone, so my weekly babysitting job ended, much to my disappointment.