On skinned knees and wing clipping

In a strange convergence of circumstances, we've had some pretty profound discussions. Maybe I'm the only one who sees the parallels. I've been brewing this post for a couple weeks now.

Last Tuesday we had our homeschooling Catholic moms' Bible study. The girls--there are three eight-year-olds--donned helmets and went riding bikes around the block. It's a small block, maybe four or five houses per side, and they've walked it before in a large group (six kids or so?). Nonetheless I was a touch nervous as Madeleine hasn't done much turning on her bike. Of course, she returned with a skinned knee.
I had thought of this before. Knees covered in Band-aids are a staple of childhood, like splinters and mosquito bites. To attempt to prevent them, protect them completely from those things.... Down that road lay madness.
She didn't cry as I washed it, dabbed it with ointment, and bandaged it though there were some deep shuddering sighs and pinkish around the eyes. She did inform me later that it hurt some as we knelt for bedtime prayers; I figured some bruising on impact made sense and gave her a pass for a few days. Life goes on.

I've since finished reading E. B. White's Trumpet of the Swan during quiet time (Lou's naptime) as well. There is a chapter entitled "Freedom" near the end, where Louis (the main swan) and his love are offered perfect--and permanent--safety at a zoo. They will always have enough to eat, no danger of otter, fox, or other predators. The cost is Serena is pinioned--she gets a wing clipped and can no longer fly. Louis chooses freedom, the ability to go from the pond in Canada to the lake in Montana as the seasons dictate. He recognizes and accepts the risk of being hunted or not having enough food. It seems a simple enough choice. I pointed that dilemma out to the kids; they seemed thoughtful.

Also a couple weeks ago, as some readers already know, Dale's and my 11-year-old nephew was set upon by a dozen thugs who did enough damage to send him to the hospital. This is not a boy who shoots his mouth off or goes looking for trouble; he wasn't out after dark or in what would be recognized as a troubled area. He was walking around his neighborhood on a Sunday afternoon with his 9-year-old sister.
I was asked, "Where were any adults?" My guess is at home though their father may have been at work; perhaps one was grocery shopping. I'm not sure exactly where they should have been; to me, it seems a bit extreme to escort an 11-year-old and a 9-year-old around their own neighborhood.

Or is it? Since her birthday, coincidentally the same day as the assault, Madeleine has been permitted to walk the quarter mile to the party store on the corner alone. She's gone twice to get a gallon of milk. Yes, both times I watched the clock for the 20 minutes she was gone, keeping an ear cocked for her return while getting Lou down for his nap.
Am I leaving her vulnerable to such an attack? Should I be accompanying her still, an 8-year-old and me at over 8 months pregnant? When does "protective" become "paranoid"? Where is the line between clipping her wings and responsible parenting, and how many skinned knees will we have before she can fly?