Boy, 6, serious after quad bike smash

A six-year-old has serious head injuries after smashing a quad bike into a car in Sydney's southwest.

The boy was riding around a Chester Hill cul-de-sac on Tuesday evening when he rear-ended the car parked in a driveway, police said.

Police are talking to the boy's family in the course of their investigations.

He remains in a serious condition at Westmead Children's Hospital.