An eight-year-old boy was seriously injured and a three-year-old was also hurt when a car reversed into them as they sat at the edge of a pavement.
A Northamptonshire Police spokesman said the youngsters were sitting on the kerb in Wellingborough when they were hit by a Ford Escort.
They were both taken to Kettering General Hospital, where the older boy was undergoing treatment for serious injuries to his head, shoulder, pelvis and arm.
The accident happened at about 1945 BST on Tuesday in Ashfield Road.
The three-year-old boy received minor cuts and bruises.
Police said they had arrested a 25-year-old man on suspicion of dangerous driving but later released him on bail pending further inquiries.