This is such a poignant post getting at an issue all parents face about boundaries and control.  My older daughter is 6, and we're getting to those questions of where can she ride her bike without us, how far can she go, who's house she can go in.  Part of it is safety, sure, but it's also for our own comfort--if we can see her, we don't have to worry as much.  It's hard to let your kid out of sight to explore wherever she wants to go, but like you say, they need to explore and learn on their own.