The flight had to divert to Shannon, Ireland on Saturday after the child suffered a medical problem.
There was a doctor and nurse aboard the plane and they helped the flight crew to assist the girl.
When the plane landed, emergency crews met the aircraft but Air Canada said that local medical authorities pronounced the girl dead.
The flight with 230 passengers continued on to London.
The Irish Sun said that a post-mortem examination will be performed to determine the cause of death.
The child's nationality has not been immediately confirmed.