I have not read through this entire thread yet, so forgive me if I am being redundant or appear insensitive. I'm just giving an opinion based on life experience.
When DW and myself were children we were never leashed. I don't even think they existed then (for kids, anyway). Mom and dad just plain and simply looked after all 4 of us (DW came from a 4-sib family too) and when we reached a certain age we were taught the basics - don't take candy from strangers, look both ways , then look both ways again, before crossing the street, if you play with crap don't act surprised when you get some on you, etc.
Even so, experience is usually the best teacher. We used to disappear into the foothills in the morning on our bikes for the entire day on a weekend or during the summer with only some lawn mowing money in our pockets to spend on food or candy and not come home 'til dinner. And, we lived in NoCal at the time ('68-'76) - the time of hippies, Manson, the Zodiac Killer, etc. Even so, it was a different time.
Fast forward to our children...
We looked after them and taught them as DW and myself were looked after and taught. We never leashed.
Our 3 kiddos are 3 years and 2 months apart in age. When they were little, it was like trying (and I can't emphasize "trying" enough
) to herd cats. Add to that the fact that our middle child (DS) was born at 1 lb. 4 oz. and has been diagnosed on the autism spectrum as well as with asperger's, but is otherwise, thankfully, as healthy as a horse, and it could be quite off-the-chain sometimes. But, we still never leashed. We just had the parental radar at full, fun, tilt all the time and we all have the best memories of all the things we've done together.
We continued to instill common sense in them as they grew and approached driving age. As my mother and father taught me, drive like everyone else on the road is a complete and total a-hole and you'll be just fine. If you're doin' 60 mph and wondering if that person at the stop sign at the cross road doesn't "see" you and is going to pull out in front of you, bet your they don't "see" you and prepare accordingly. A green light does NOT mean go, it means look both ways, then look both ways again, then go.
I hear about and see so much "helicopter" parenting these days, and it seems to be approaching new levels. It is approaching more like what I would term "force field/bubble parenting".
Ultimately, to each couple/parent their own. But, seems to me anyway that kids, no matter what their situation, need to be just that...kids.