The incident happened Saturday night on the westbound side of the 60 Freeway near Vineyard Avenue.
The 4-year-old died at the scene. Medics took the woman to the hospital, where she later died.
The other child, 1, remains in critical condition as of Sunday morning.
It appears the driver of a U-Haul truck hit their car as it sat stalled in the middle of the freeway.
Investigators say that driver is cooperating and that alcohol does not appear to be a factor in the crash.