UPDATE 10.15AM:  A THANKFUL Blacks Beach mother whose children had locked themselves in the family car has praised the quick actions of emergency services.

It was 29 degrees in Mackay when Shannah Rumble's children Corben, 7 months, and Slade, 2 years, accidentally trapped themselves in the car.

"I put them in the car and the keys in the boot - and then the little one has pressed the button," Ms Rumble said.

The reaction from emergency services was immediate, she said. 

Because police and two RACQ crews rushed to the CBD business car park, she said, her children were only stuck for 15 minutes.

While Ms Rumble said she was a little shaken by the incident, she said her children were calm.

"They had bottles and a milkshake in the car so they were fine," she said. 

But the car key accident could have ended much worse a Queensland Ambulance spokesman said. 

"On a hot day, the inside of a vehicle can reach up to 40 degrees hotter than the outside temperature," he said.

"Most of this rise in temperature occurs within the first five minutes, meaning it doesn't take long for a child to feel the effects of the heat.

"There is no safe amount of time to leave a child alone in a car."

If a young child does begin to show signs of a heat-related illness, he said it was best to cool them down in a cool shower or bath.

INITIAL: MACKAY police are responding to reports of two children locked in a car in the Mackay CBD. 

Officers were dispatched to the job just after 9.30am after a person called to report the two young children were locked inside a Prado. 

It is understood the children have been in the car for a short period and have access to liquids. 

Police are on scene with an off-duty paramedic.