The police serious crash unit is investigating the incident.
Lillian Bond lives across the road from where the accident occurred and was the first to call 111.
"I heard a loud bang and then a child screaming. It was a freak of an accident," Ms Bond said.
"The car wasn't going any more than 5km/h as it backed out of the driveway, but the boy was really stuck under it after it hit him.
"I heard the scream and looked out my window. It looked like he was trying to catch up to his sister, who was in front of him."
Ms Bond said it took half an hour for the boy to be freed.
"They were using trolley jacks and wedges to try to get him out.
"I had to move away from the window after a while; I didn't want to watch anymore."