The crash was reported around 12:35 p.m. at the intersection of Domenigoni Parkway and State Street.
Cal Fire Capt. John Clingingsmith Jr. said firefighters were responding to the Cranston Fire in the San Bernardino National Forest when the fire engine and sedan collided.
The fire engine reportedly had its emergency lights and siren activated at the time of the crash.
The 43-year-old driver of the Mini Cooper died at the scene, the California Highway Patrol said. A 10-year-old boy, the front passenger in the sedan, died at a hospital.
Another child in the back seat was taken to a hospital as a precaution. CHP said everyone in the Mini Cooper had their seatbelts on.
Three firefighters were also taken to a hospital for further evaluation, but were later released.
CHP is handling the investigation.
Investigators do not believe drugs or alcohol contributed to the crash, which remains under investigation by CHP.