The 8-year-old and 9-month-old were found unconscious around 4:35 p.m., according to authorities.
Their mother was in a panic, neighbor Michael Sam said.
"She was like, 'my baby, my baby' and we just went down there to see what was wrong," Sam said.
The older child was worse off, he added.
"She couldn't respond to nothing we were saying. Like, she couldn't hear us," Sam said.
Sam helped get the children out of the vehicle as first responders arrived. Pearland police said a 13-year-old called 911.
"We later learned the vehicle had been left running in a garage presumably so the car could cool down," Capt. Onesimo Lopez said.
The children were rushed to the hospital with carbon monoxide poisoning. Police are not sure how long they were in the vehicle with the door shut.
"One shouldn't leave a car running in a closed garage. It's a recipe for disaster," Lopez said.
Police said it appeared this was an accident.
The children are recovering at the hospital and are expected to be okay.
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