The victims' identities were not immediately released, pending notification of their relatives, reports say the child was under 10 years old. Details on the cause of the accident were not immediately available.
East Contra Costa County fire officials say the crash was reported about 3:19 p.m. on Byron Highway and Clifton Court.
California Highway Patrol officials say the road will remain closed as crews investigate the accident.
CHP says the collision also caused a diesel spill over the California Aqueduct, and Caltrans crews are on scene cleaning it up.
Traffic was mounting near the scene Friday evening, and CHP officials say there is no estimated time for reopening.