The collision involving as many as four vehicles occurred in the southbound lanes near the 14 Freeway just after midnight amid a downpour.
The driver of a red Mazda pickup lost control, spun into the center divider and overturned, according to California Highway Patrol officer John Lutz. A Nissan Altima carrying six people struck the pickup and was then rear-ended by a Chevrolet Suburban.
"[The truck] actually slid out into the middle of the freeway and a couple other vehicles collided with it, including that Nissan and the Chevy Suburban," said Lutz.
The driver of the overturned truck, identified as 53-year-old John J. Blackburn, of Frazier Park, was pronounced dead at the scene.
The impact from the crash pushed the Nissan's trunk into the rear seat, and all victims had to be extricated from the vehicle. Because of the extensive damage to the sedan, it's unclear if the four children had seat belts on or not.
Jessica Milan, a 27-year-old passenger in the Nissan, was transported to Henry Mayo Newhall Memorial Hospital in Santa Clarita with major injuries. Her three children and nephew in the back seat died. Those children were identified as Xochitl M. Hernandez, 5, Edgar A. Hernandez, 8, Cynthia J. Hernandez, 9, all of Whittier and 12-year-old Luis Villegas of Oakland.
The driver of the Nissan, 25-year-old Francisco Enciso, of Daly City, suffered major injuries and was transported to Providence Holy Cross Medical Center in San Fernando.
Four people in the Suburban, including an infant and a toddler, suffered minor injuries and were taken to hospitals for treatment.
CHP officials said the weather was a contributing factor in the crash, but they're also investigating the possibility of unsafe speed or a mechanical problem.
The crash closed all southbound auto lanes for about nine hours during the morning commute, and diverted all Los Angeles-bound traffic to the parallel two-lane truck bypass through the pass. The closure backed up traffic for miles on the busy freeway and and the Old Road.
All lanes were reponed just after 8:30 a.m.
A memorial account has been set up to help with funeral costs for the children who died in the crash. If you would like to help you can send money to Wells Fargo Bank, account number 316-479-3865.
The Associated Press contributed to this report.