A quirky thought: I'm very much an advocate of "free range parenting", and mostly grew up with it myself in the US, and it's what my kids have here in Germany. My 7 year old walks to school alone (in my neighborhood), my 10 year old takes the subway to school, and they have a large degree of freedom in our neighborhood, generally going to after-school activities on their own.

But I wonder if part of why people worried less in earlier generations is that we were so close to the time where childhood actually was dangerous: 100 years ago in the US, 20% of kids didn't live to adulthood (mostly because of diseases we can now prevent). I wonder if that had some cultural impact on perception of relative dangers.