Andrea Brown's son, Cameron, was found crying by another parent at a busy junction at Barry Docks Link Road in Barry, Glamorgan after he slipped unnoticed off the vehicle.
His outraged mum had been waiting in her car nearly half a mile away at the next bus stop and was concerned when her other sons, Elliot, nine, and Alex, seven, got off the bus without Cameron.
Ms Brown, 40, told Wales Online: "When the bus turned up and the courier let my two children off and one was missing, I was distraught." She raced to the previous stop where she found Cameron crying but with another child's parent.
Ms Brown alleges that the school-bus escorts who are employed to supervise the youngsters did not ensure there was a parent present to collect Cameron when he got off the coach. She also says it is not the first time it has happened, and that other children have got off the buses unsupervised.
A spokesperson for Vale of Glamorgan council and service operator EST Coach said they were looking into the incident, and stressed that safety was their highest priority, but Ms Brown said she was not happy about the response she had been given.
"I haven't received an apology. I haven't been told anything. They were just querying why his brothers didn't look after him, which I think is wrong because they are seven and nine years old," she said.
She has now refused to allow her children back on the school bus since the incident.
Ceri Edwards, the council's senior transportation officer, said: "Our policy states that, at the home end of the journey 'Primary school pupils should only be set down at their normal stop in the afternoons unless there has been a specific request from a parent/guardian directly or via the school'.
"I would add that we would normally expect parents to be at the stop to collect their children off primary school transport. Also in our Contract Conditions for School Transport provision we state that where the parent or carer is not at the stop for the afternoon journey then the pupil should be taken to either a) the bus depot, b) a police station or c) returned to school – the most appropriate course of action to be decided by the driver."
How scary for the mum AND the little boy - and worrying that a four-year-old could jump off with no one noticing!