Around 6 p.m. police responded to a call that a child had been struck by a vehicle on Harvest Road.
Responding officers discovered an unoccupied car that had been parked without the emergency brake engaged started rolling backward down the driveway where it rolled over the boy's thigh, police said.
A parent quickly pulled the boy away from the car before the front tire rolled over the boy, police said.
The 3-year-old was taken to Pleasanton's Amador Valley High School by ambulance where a medical helicopter flew him to Stanford Hospital in Palo Alto, where he was treated for injuries that were reportedly not serious, according to police.
A second child, an 18-month-old girl, was also near the car and was knocked over. She suffered minor scrapes, and medical personnel on the scene determined she was uninjured.
Police said it is unclear if the girl was pushed back because of the rolling vehicle or if she fell over when the parent grabbed the boy from under the car.
Police reminded parents to set the parking brake in vehicles parked in residential areas where pets and small children are present.