Children killed by heatstroke after their mum locked them in a car.

Two children found dead in a hot car after being locked in by their mother were killed by heatstroke, a coroner has ruled.

Cynthia Randolph, 25, is facing criminal charges over the tragic deaths of daughter Juliet, two, and 16-month-old Cavanaugh.

She initially told officers the pair "took off" and by the time she found them they had locked themselves in her car.

But police in Weatherford, Texas, say her story quickly unraveled and she later revealed the horrifying truth.

Juliet had refused to get out the car when the family had returned from a trip and Cynthia decided to lock both in to 'teach them a lesson'.Cynthia then went off and smoked marijuana and took a nap for two to three hours,Daily Mirror reports.

In a police interview, Cynthia said she believed the pair would eventually get out the car when they were ready.

Temperatures outside had hit 35 degrees when the events unfolded on May 26