The six-year-old boy was involved in a multi-vehicle pile-up which happened at the intersection of Dixon and Mandurah roads in East Rockingham at 3pm on Friday.
Police said a Holden Cruze collided with the rear of a Kia Carnival, which resulted with the cars colliding with another two vehicles and a bus.
The boy was was flown to the Perth Children's Hospital in a critical condition.
Police confirmed this morning the six-year-old had died.
WA Police Commissioner Chris Dawson said police were investigating whether a medical episode sparked the chain reaction.
A 64-year-old male, was taken to Rockingham Hospital for treatment.
He has now been discharged.