I know the discussion of urbanism vs suburbanism is a common topic on HN, but I don't think suburbs themselves are the root issue here.

When I was growing up in the suburb, there were kids outside all the time. Yes, some friends lived across town in another suburb, but we just biked there instead of walking.

Now when I visit that same suburb, there are no kids in sight. I still see adults of parenting age, so I assume there are still children in the neighborhood, but they're just indoors. The density of the town didn't change, but rather people's attitudes towards where kids can and can't be seem to be what changed. I also suspect the declining birthrate and having fewer kids is contributing to the problem too.