Not sure how to respond to an inappropriate game at a playdate

My son (5 YO) has a friend in a nearby neighborhood who is a little older (7). They have been outdoor friends--they met randomly at the park, hit it off, and then played outside together for most of the summer and then on weekends once the school year started. This weekend the weather turned cold, though, and since it was too cold to play outside and both boys wanted to see each other, the other boy's mom offered to host them playing there. As a single dad with full custody and a full-time job, I jumped at the chance to get a bit of cleaning done.After I picked my son up we were talking about what they did and he said (among other things) they played hide and seek and once they'd been found they'd have to pull their pants down. I was disturbed but didn't want to let on because I didn't want him to clam up while I found a way to ask some follow up questions for more information. I learned that they played this a total of two or three times, that it was just pants and not underwear, and that it was just the two of them playing. I was clear to him that this is not an appropriate variation of hide and seek, and we reinforced some previously-established ideas about the parts of the body underneath one's pants being private, what to do if anybody or anything makes you feel uncomfortable, and so on …But beyond that I don't have a sense of how this fits on the scale of fairly standard inappropriate behavior to disturbingly inappropriate, and so I'm not sure how else to respond. I'm not close with the other boy's parents--we only met because our sons became friends. This makes me uncomfortable with the idea of letting him go over again, which basically means (with cold weather regularly upon us) an end to their playing together, and I'm not sure how best to explain that to my son. I'm also not sure whether this rises to the level of something that I need to share with my ex-partner (who tries to see our son when she can but is not much help as a co-parent and winds up spending only a few minutes here and there after school with him ,along with a weekend day out together once every two or three weeks).Any help/advice/perspective would be appreciated. via /r/Parenting https://ift.tt/2CJOeVv