The family were in a Honda sedan that was struck by a van, police said. The driver of the van, who suffered minor injuries, apparently collided with the Honda and his vehicle, then flipped over and came down on top of the car.
The crash occurred at about 7 a.m. Sunday in Hesperia, east of Los Angeles. Investigators believe the family was headed to Disneyland to celebrate the girl's birthday.
Police had not released the identities of the family late Sunday. The mother was described as 25 and the two boys 9 and 2.
The van driver was treated at a hospital. No charges had been brought late Sunday, but the San Bernardino County sheriff's department said the investigation was continuing.