The Whittier Police Department says a vehicle was exiting a building when the driver lost control and hit the girl Sunday evening in Santa Fe Springs.
The girl, who was not identified, was killed instantly. Police say the investigation is ongoing.
KABC-TV reports the show was inside the warehouse of Kool Aid Hydraulics, a firm that outfits cars with custom aftermarket parts and accessories.
The station says hundreds of people regularly attend monthly car shows at the warehouse.
Santa Fe Springs is about 15 southeast of downtown Los Angeles.