Police said the 22-month-old girl was taken to Yalgoo Nursing Post but died from her injuries.
The accident happened on Geraldton-Mount Magnet Road between Yalgoo and Mullewa about 11am.
Police said the 43-year-old female driver of a silver/beige Toyota Prado lost control of the car, which veered from the road and rolled over.
Three adults and two other children in the vehicle suffered minor injuries.
Anyone who saw the crash or the Toyota Prado before the crash is asked to call Crime Stoppers on 1800 333 000 or make a report online.
This article originally appeared on The West Australian.