According to the Montgomery County school website, having the kids walk a mile with a sibling is within normal community standards, and in line with guidelines set forth by the county itself.
(See www.montgomeryschoolsmd.org/parents/basics/transportation/ )
In Montgomery County where this occurred, school bus transportation is only provided for elementary school children who live further than 1mi from school, and for middle schoolers (11yo+) further than 1.5mi. The county's guidance for elementary school kids walking 1 mile or less is "Younger walkers are encouraged to walk to and from school with siblings, older children from their neighborhood, or parents. At many schools, Montgomery County crossing guards help walkers cross at busy intersections near the school. In most elementary schools, student safety patrols guide younger children in crossing smaller neighborhood streets."
I don't see how CPS has a leg to stand on here; the children were simply practicing what they are expected to do by the county school system itself.