The California Highway Patrol identified the driver as a 26-year-old woman. She was driving north on the 710 Freeway on Monday with her four children when the collision occurred.
Authorities say a black Honda SUV lost control for unknown reasons, went off the side of the roadway, flew over a fence and slammed into a tree.
Two of children, girls ages 4 and 6, died at the scene. Their 6- and 8-year-old brothers remain in the hospital, one in critical condition.
Investigators say excessive speed may have been a factor.