The group was crossing at an intersection of Cardinia Road and Princes Highway at Pakenham on Friday, just after school had finished for the day.

"A woman aged in her 30s has chest, back and shoulder injuries. She was taken to The Alfred hospital in a serious condition," Ambulance Victoria said.

"A primary school-aged girl has abdominal and leg injuries and was taken to the Royal Children's hospital in a stable condition."

Five other girls suffered cuts and grazes. Of those, three were taken to Dandenong hospital and two to Monash Medical Centre.

A boy was treated at the scene but wasn't taken to hospital.

"Police are currently searching for the driver who left the scene on foot," Victoria Police said in a statement.

Police described the driver as of Pacific Islander/Maori appearance, aged in his mid-20s with a solid build with black afro-style hair.

He was last seen running west on Princes Highway wearing a dark blue t-shirt and long black pants.