Boy, 6, critical after car hit in Sydney's west

A six-year-old boy is in a critical condition with head injuries after being hit by a car in Sydney's west.

A rescue helicopter and several ambulances are on their way to Blacktown, where the boy was hit at 3.10pm on Wednesday afternoon.

"It's very serious," an ambulance spokeswoman told AAP.

No one else was injured.

A two-year-old was earlier hit by a car at Haberfield in the city's inner west just before 3pm but wasn't badly hurt.