Every town/city in the US is different as far as risk. That said, the risk that a child will be kidnapped is not zero.
My daughter was kidnapped*. She was 10 years old at the time. I feel I have "some" input to offer...
I feel the parents were stupid to allow their children at this age to walk that distance alone. My opinion of this can be argued to be colored by the events in my families life, yes. Yet I believe I'm right on this issue. I also believe that the decision to allow their children to do this was THEIR choice. The kids were together which is a plus. The eldest was 10 years old which fills me with concern. I do not believe they irresponsibility put their children in danger to the level of calling CPS on them. This was just over kill.
Background: At 10 years old I was getting up at 4:30 in the morning to deliver newspapers (1980's Los Angeles County). I would never allow my children to do this today. I also walked and/or rode a bike to/from school from 3rd grade-9th grade. The distances were all less than 2 miles. I got a ride TO high school (10th-12th) and took a bus back -- it was a hike. I'd be hard pressed to allow my kids to go to/from school on their own before high-school.
*My daughter was recovered some 12 hours later alive. The monster who took her is facing 3 life sentences + 300+ years on various counts and has yet to go to trial (probably will happen within the next 3-4 months)