The incident occurred on the 25800 block of Pacific Coast Highway near the Malibu RV Park around 8:30 p.m.
Authorities said the victims were coming back from the beach when they were struck while crossing the highway.
"We understand that the victims were crossing PCH from the ocean side to the other side," said Los Angeles County sheriff's Lt. Patrick Mathers. "They were legally crossing the highway. We believe they were possibly struck by the side view mirror."
The adult victim sustained a broken arm. The 3-year-old sustained a fractured hip and numerous facial abrasions. Both victims were transported by helicopter to Ronald Reagan UCLA Medical Center for treatment.
The incident shut down Pacific Coast Highway between Corral Canyon Road and Puerco Canyon Road for about 90 minutes Friday night as deputies conducted their investigation.
The hit-and-run driver's car has been described as a white or silver-colored SUV.
Anyone with information regarding this incident was urged to contact Lost Hills Sheriff's station at (818) 878-1808.