In car-centric suburbia, there's always traffic at the same times every day, and often poor infrastructure for biking.

100% Most of the roads seem designed to prevent it.

I grew up in a small town and we all took bikes to school or walked the ~2 miles. The drivers seemed more aware that they didn't dominate the road.

Yep. It would probably be fine and I'd let my hypothetical kid do it anyway, but the fact remains that the biggest risk along the way is.. car traffic. Parents stressed about getting their kid to school and then to work on time. Of course there's winter and that commute becomes even more insane by bike or car.

Just let kids ride slower on the sidewalks and follow pedestrian street crossing rules.

That's treating the symptoms not the cause. The roads should be safe enough for kids to ride on.

Intersections are where many pedestrian injuries and fatalities happen, due to "inattentive" motorists. The reality is the motorists are probably attentive to other vehicular traffic, which poses a much greater risk to them but poses a huge risk to pedestrians.

Then there are the agressive motorists. I have seen motorists intentionally[1] accelerate towards school crossing guards. People seem to get impatient when being held up in traffic, even when it is a product of their own decisions (like driving past a school on a residential side street when school is starting for the day).

[1] Of course, I don't know their actual intentions. Yet I have difficulty interpreting a vehicle that is creeping slowly into an intersection then suddenly accelerating towards a safety vested person as anything other than a deliberate act.

Very few sidewalks (at the time) in our area (unincorporated area of the county). Lots of should er space though.