Port Hedland police are trying to determine the cause of a car crash which left two children with serious injuries.

The children, aged six and seven, were travelling with two women in Cook Point yesterday when the car struck a light pole.

One of the children, a girl, suffered serious head injuries.

The Royal Flying Doctor Service flew the women and children to Perth overnight.

Sergeant Richard Moore says police will investigate whether the children were wearing seatbelts.

"We understand the driver and the passenger at the front were wearing a seatbelt," he said.

"We're making an inquiry in regards to the young girl in the back seat to whether she was wearing a seatbelt."

The children remain in a critical condition.