It happened around 6:30 p.m. Monday near the intersection of 52nd and Chancellor streets in West Philadelphia.
The boy's father told police he stopped at a restaurant where he delivers items. He left his Honda CRV running with the 4-year-old child asleep in the car seat.
That's when investigators say a man jumped into the driver's seat and drove off.
A witness started banging on the window telling the suspect a child was in the back. Police say the witness was able to help stop an 18-year-old male accomplice from jumping into the vehicle.
Officers found the vehicle two hours after the theft in a parking lot at University Avenue and Curie Boulevard, which is about two miles from the initial scene.
The suspect who stole the vehicle fled the scene, but the child was found in his car seat unharmed.
"I personally saw the child and the child's in good spirits, drinking and eating right now," said Philadelphia Police Chief Inspector Scott Small.
Police believe this was a crime of opportunity and the man may not have known a child was in the car.
The 18-year-old who was detained is cooperating with police but could also face charges.
Investigators are confident they will find the person who stole the vehicle.
Anyone with information is asked to call police at 215-686-TIPS.
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