SS was way over the top in this instance. I get the feeling had these children been walking through a 'naice' area the OP might not have been so quick to involved them rather than going through the school...and unless I've missed it I don't understand how the OP knew which school they attended, esp as it doesn't appear to be the one her own children attend?
Children do, unbelieveably, walk to school on their own. As a primary age child I was the only child in my school who didn't walk to school on their own/with siblings/with friends. Less than a generation ago it was the norm. In the last 10-15 years with people living further from schools it has become a rarity.
From what has been said about these children, at the very most a call to the school was appropriate. Anything more is going over the top.
My DS2 (in Yr 5, will be 10 in April but the size of a 6-7 year old and with a young round face which probably makes him look younger than that) walks with his brother (Yr8) half the way to school, then walks the second half on his own.
I would be livid if someone reported him for doing so, because of the wider ramifications, not just for us as a family, but for me professionally, and as I know what mileage my Ex would make from it.
Threads like this frankly make me concerned about any parenting choice I make being subject to SS scruting at the whim of a passerby with no information.
It worries me that the criteria of what justifies to many people a report to SS seems to be growing ever wider.