ETA relevant links: https://youtube.com/@NotJustBikes https://letgrow.org/
ETA again: I glibly mentioned "being killed by a driver" but of course navigating the typical US built environment if you're under 16 or otherwise unable to drive is a miserable experience in a number of ways even if you survive it. Highways make pedestrian paths unnecessarily roundabout. Parking lots make everything further from everything else. Crossing major roads requires getting drivers to notice and stop for you (harder when you're short!), or waiting through interminable signal cycles, etc.
It may depends on where in the US you're talking about, but in my area none of this is actually true. Although lots of people believe it is.
I do worry about car on pedestrian crashes but the city has crossing guards and traffic calming for just this.
Where are these places that you can’t be outside as a child?
[1] https://www.google.co.nz/maps/@33.8644514,-84.5949946,3a,75y...
[2] https://www.google.co.nz/maps/@-41.2953813,174.7673872,3a,75...
edit: Might add that close to those shops in [2] there is a large botanic garden with play area and stream, one small and one very large reserve with native bush and an abundance of mountain bike tracks, a playing field, the central city area with cafes, many theatres, galleries, a library, and all sorts of other things that a child can make use of without supervision. All of this is within 5 minutes walk of those shops. Within 10 minutes walk there is the waterfront and a very safe swimming beach.