You can if you all know each other in the neighborhood and have no problem with kids coming in and out. We don't ask each and every time at all. Just not necessary if you know each other. No "arranging" necessary. We were just a neighborhood where we all would sit outside in the evening, or work in the yard, while the kids ran around and got to know each other. They ran in and out of each other's houses and walked to the park on the next street together. Much depends on your neighborhood and just getting outside with the kids. Kids have a way of finding each other if given the opportunity. And then it's up to the parents to allow them a bit of room to just be kids. Again, I realize that is contingent on the neighborhood; we were fortunate to live in very kid-friendly neighborhoods where people just kind of got to know each other very informally and felt safe allowing their kids to be around each other.