At about midnight on Sunday a Holden Commodore towing a trailer ran out of petrol on the Brookton-Corrigin Road, about 28 kilometres west of Corrigin.
A 27-year-old man left the car to walk towards Corrigin to find a petrol station while a woman and four children remained.
A short time later a truck towing grain trailers came around a bend in the road and crashed into the parked car and trailer, pushing them into a nearby paddock.
The 26-year-old woman and 10-year-old boy were killed in the crash.
Two boys aged eight and five were seriously injured while a three-year-old girl received minor injuries
All three were flown to Perth Children’s Hospital by the RAC Rescue Helicopter.
The Brookton-Corrigin Road is currently closed as major crash detectives investigate the incident.