Police said the collision happened near the intersection of Santa Monica Drive and Victoria Avenue just after 7 p.m.
Police said a silver sedan was heading northbound on Victoria Avenue when it veered into a bicycle lane where the children, all girls, were walking in.
The children were ages 10, 8 and 6. The 10 and 8 year olds suffered moderate injuries, but the 6 year old was seriously injured.
All three victims are expected to survive.
A mother of two of the girls was with the group, but was not injured.
The California Highway Patrol said they're looking a silver sedan with front left damage.