Chino police spokeswoman Tamrin Olden said the rifle was mounted on a police motorcycle that was on display during the Newman Elementary School's Red Ribbon Week activities Wednesday.
During the officer's presentation of the motorcycle, a child approached the vehicle and pulled the trigger on the rifle, police said.
The bullet struck a metal plate on the weapons mount -- a safety measure -- and metal debris from the impact hit two of the children, who were taken to the local hospital with minor injuries, police said. A third child was treated on the scene and handed over to parents, the Inland Valley Daily Bulletin reported.
"We are looking into exactly what led to the accidental discharge," said Julie Gobin, a school district spokeswoman. "There are security measures in place to prevent that, but that's part of the investigation."