LOTS of Moms do that.
You are not the only one.
USUALLY, if a child is not picked up.. or the Mom is late... the child is to go to the school Office. And someone there will call the parent.
There are sometimes LOTS of kids, waiting in the Office, because parents forget, Wednesdays, are earlier pick up time.
Now, at my Daughter's school, NO ONE is allowed to drive or pick up another child, UNLESS the parent has authorized it (per the school forms that the parent fills out)... OR unless the parent was called, and the other Adult got PERMISSION to take that child home.
Now, the Teachers at my Daughter's school, know how a child gets home. AND if a parent is late.... the child is to go to the school Office and to tell them.
In your case, this happened at the bust stop, once your child debarked from the bus.
So yes, good thing, another Mom noticed. She was not at the school by then.
BUT, does your child KNOW your cell phone or home phone number???
What if a Stranger, picked up your child at the bus stop?????
I am surprised... that they let your child off the bus, with no parent there.
AND... what if a parent never shows up or is very late???
What then?
What are the school's or Bus rules???
I would ask the school.