The crash happened at Bristol and 10th streets as the two were crossing the street with an adult and five other children. It was not immediately known if they were in a crosswalk.
Acccording to the Orange County Fire Authority, the 11-year-old girl had no pulse and was not breathing when she was transported to UCI Medical Center. Her 8-year-old brother was transported to the hospital in critical condition.
Authorities said the driver did not flee the scene. The cause of the crash remains under investigation.