CHP said at 10:30 a.m. it was searching for the 30-year-old male driver, who fled the scene.
Four other females inside the car were transported to John Muir Medical Center in Walnut Creek with major injuries.
CHP received the initial report at 5:14 a.m. about the crash. The gray Mazda CX-9 was traveling northbound when it traveled through the dirt median separating the two freeways and overturned several times, landing on its roof.
The 9-year-old was pronounced dead at the scene. CHP said "several" occupants in the vehicle weren't wearing seatbelts.
CHP said anyone with information about the crash, or the whereabouts of the 30-year-old Hispanic male driver, can call the Contra Costa CHP at (925) 646-4980 or email contact information to 320investigations@chp.ca.gov to be contacted by the investigating officer.