I guess I assume that if you have a rapport with a neighbor, you can trust that they won't be out to screw you if a kid gets hurt, but maybe that's naive. Or maybe my parents were just lax; I remember my grandparents let us ride around on a moped that could do ~35 mph when I was probably under 10, and we'd go wander around in a forest near their house, or me and the neighbor kids would go play in the desert near my house growing up when I was probably ~6. Probably never more than .5-1 miles away, but still pretty much only "supervised" by older kids.