Police say a group of children may have witnessed a collision at a caravan park in which a seven-year-old boy was hit by a car and later died.
A teenage boy is being questioned by officers after the boy was hit by a BMW 3 series estate at the Shirenewton site, east of Cardiff, at about 15:00 GMT on Saturday.
The boy, 16, was arrested on suspicion of causing death by dangerous driving.
Police are urging the witnesses or their parents to come forward.
South Wales Police said: "Police investigating a fatal road traffic collision on the Shirenewton caravan site are reappealing for information.
"It is believed a group of children and young people may have witnessed the collision and officers are urging them and their parents to make contact.
"They might have important information to assist the investigation and would also be offered any support they might need."
The seven-year-old hit by the car was taken to the University Hospital of Wales in Cardiff where he later died.
"A 16-year-old youth from the area, who is believed to have been driving the BMW at the time, has been arrested on suspicion of causing death by dangerous driving and is in custody and inquiries are ongoing," said police.
Anyone with information is asked to contact South Wales Police on 101 or Crimestoppers anonymously on 0800 555 111.