The children had left their car that had stopped on the side of the road to retrieve luggage that had flown off.
The tragedy happened around 6 p.m. on Sunday on the eastbound side of the freeway.
The children are believed to have been under the age of 12 and were hit by cars going at full speed, according to the Vista Fire Department.
The mother was driving the car, and was later arrested for a DUI, CHP confirmed to KGTV.
It is also believed that the mother stayed in the car, at least initially.