In theory I agree. But in practice, where I grew up, it was pure suburbia. I lived a few miles from school. No sidewalks, and roads very car centric. In elementary and Jr high I used to bike to school, regularly, and I don't remember it being abnormal; I remember a lot of kids doing it. I remember my dad commenting on it being weird that there was a crossing guard we had to wait for on our school corner (which crossed a 4 lane busy-ish street) on one of the rare days where he picked us up in his car.
I think a big part of this is just our risk tolerances change. We naturally want to protect kids, and looking historically we've taken safety to relative extremes compare to 20 years ago (and similarly before then). Its natural to want more security, and its obvious how it helps. But its not as obvious what we've given up in return.
Interestingly, I moved to Portland recently, and live mid way between suburbs and downtown; its fairly dense but mostly SFH or townhomes in my neighborhood, many businesses, some homeless. Kids of all ages walk to school and are... just everywhere, around here. Its quite normal to see people walking with their kids too. Its just got a really great mix of urban density -- you can walk to stores, bars, school. There's a few high traffic streets, but they are not large and the speed limit is 20mph. I think what this neighborhood has taught me is, while our risk tolerances are lower than ever, its still a bit of a design and culture choice to not let kids walk around. Its nice to see here, in Portland of all places, there's a bit of a counter trend going on. I hope its what the future looks like in other places (here and elsewhere) too.