Yeah... but you could argue the parent isn't responsible for the children at that moment if they're not there.
I'm wondering if the mode of transport makes a difference here? Would people (you, reader!) have a different opinion if the kids were not travelling by bus but by train, taxi, plane, ferry, or just walking to school?
IMHO▸ , it depends on the degree of complexity that the child has to handle.
Train, arguably not much different from a bus, should be fine for a child.
Taxi, OK if pre booked and pre paid for by an adult, but I would be doubtful about a young child having to find or book a taxi and pay the fare on their own.
Plane, fine for a domestic flight, international travel adds more complexity than I would want a young child to handle.
Ferry, probably OK for a domestic journey, but again I would not want a young child to undertake a proper international journey alone.
Walking should be fine unless there is some local factor that renders it unusually risky.