Ambulance Victoria paramedics, CFA and the RACV attended the Harcourt BP about 2pm to rescue the trapped children.

A witness said the car locked automatically when the mother was getting petrol.

The local fire brigade smashed the window and paramedics assessed the children on scene.

It is believed the young girl's temperature rose to 39.8 degrees in 10 minutes.

The witness described the situation as "a parent's worst nightmare".

"The children were visibly agitated and it was terribly scary at the time," she said.

"I have children and I know how it must feel to have this happen. I just called everyone I could think of to help get them out."

Ambulance Victoria spokesman John Mullen said the fire brigade were able to attend the scene quickly. 

"The children were assessed on scene but did not require treatment," he said. 

"They were locked in accidentally and it just emphasises our message to take extra care.

"It shows how easily these things can happen." 

The RACV backed this message, saying disastrous things could happen in minutes. 

Road user behaviour manager Melinda Congiu said RACV roadside assistance patrols faced increased demand from members due to vehicle breakdowns in the heat, meaning the message was ever important. 

She said RACV patrols had already rescued about 50 children and pets from locked cars since January 1.

“While RACV prioritises call-outs to vehicle lock-ins, the few minutes it can take for a patrol to reach the scene can make an enormous difference when temperatures are extreme,” she said.

“It can only take a few minutes for a child locked in a car with that temperature to become seriously ill or die due to heatstroke or dehydration.”