Ambulance Victoria spokesman John Mullen said the boy was struck in Main Street shortly after 3pm.

He said the primary school-aged boy was taken to Bairnsdale hospital with serious head injuries.

A helicopter was dispatched to fly him to the Royal Children's Hospital in Melbourne.

Hospital spokeswoman Laura Richards said the boy remained in a critical condition on Saturday night.

Victoria Police spokeswoman Amelia Penhall​ said investigators were interviewing the driver of the car that struck the boy.

"The driver and passenger of the car are assisting police with inquiries," Ms Penhall said.

"Police are also reviewing CCTV footage and speaking to witnesses."

Meanwhile four people, including a preschool-aged girl, were injured in a crash at Merrijig near Mount Buller.

Ms Penhall said the four were parked on the side of Buttercup Road when their car was struck from behind about 3.30pm.

"Two people suffered minor injuries and two people suffered serious but not life-threatening injuries," she said.

The girl and a woman in her 30s were flown to hospital in Melbourne.

A man and woman, both in their 20s, were taken to Wangaratta hospital in a serious condition.

The driver of the moving car was uninjured and is assisting police with inquiries.