Mostly, my kids are each others' friends. Most of their friends - like most of mine - live at a distance.
However, a few years ago, we lived in a trailer park where there were quite a few children. One of them spent a lot of time with ours, including riding with us to church. We limited their exposure to this child, however, when we overheard him explaining to them what the term "69" means. They were not allowed to be alone with him or around him without a parent at all after that.