> I let my kids roam free here in the US (Columbia, MD), but the reality is that they don't want to because there are no other kids outside to play with.

I suspect there are no other kids outside because there are few/no desirable places for kids to congregate - places that are safe from moving cars, enforced property laws and adults with poor judgment.

My neighborhood (Bryant Woods in Columbia, MD) is honestly perfect in this regard. It's filled with walking trails and playgrounds. Lots of green space.

The roads are also twisty & turny. They're difficult to drive fast on.