That was very mature of you, especially for that age.
Well actually ZAPS, I do not recall my maturity (i.e. be nice and not a jerk) but possibly in my mind I thought if the boy was embarrassed enough and told his mom, and then his mom told my mom, she may bring up the previous episode of me wetting my sleeping bag and having to wear plastic pants over my underwear the rest of the week when I went to bed. To top it off, if the boy was really upset, my mother could have made me wear my plastic underpants to school the next day, apologize to the other boy and admit to him I was wearing plastic underpants at the time in which he could have told the other kids and I would have been horribly embarrassed. Thinking of this scenario in about 5 seconds on the playground may have been one reason I held back from being immature about the situation. Still glad to this day I did not humiliate this kid.
I recall several other times when I was around 4 - 7 seeing in a laundry basket that had just been brought up from a laundry room several washed pair of plastic underpants, snap on underwear / diapers when I went over to friends' houses. Usually the items were beneath other clothes but through the basket slats on the sides you could see them and it was obvious. Most of the time they had a younger sibling which probably explained it but two other times they were the only child and it made me wonder. But, being we were friends and I had a plastic sheet on my bed until I was 11 I never said anything ( I did not have any big episodes except when I was 8 and slept over my friends house, majorly wet his bed, and had a period after that where I needed to wear protection at night again, but this only lasted about a year). I recall asking my mom about taking the plastic sheet off my bed when I was around 10 but she said to "wait and see how things goes". Sometime when I was 11 she removed it but I do not recall asking her.
I had one friend inquire about my plastic mattress cover when I was around 9 or 10. I just replied it had always been there and it was so if I ever spilled any drink in my bed it would not stain the mattress. He seemed to think that made sense and we went on to more important things like playing a board game or something. Back then we knew some kids had accidents from time to time (stained sheets drying on the back yard line); funky smells in a bedroom, a plastic mattress cover prominently displayed before clean sheets were put on a bed, etc. With disposables today you so not see this much anymore since no laundry of any obvious bed-wetting undergarments.
