The child was declared dead at the scene in the city near Manchester.
Rescue services were called at around 10:40 am on Monton Road in the Monton suburb, after a collision between a blue Vauxhall Corsa and a pedestrian had been reported.
A 24-year-old man was arrested on suspicion of death from dangerous driving and is currently being held in custody, Greater Manchester police said.
Sergeant Lee Westhead said, "Tragically, a small child has lost his life this morning and we're doing everything we can to support your family at this devastating time.
"I would ask anyone who has witnessed the collision, or anyone with information, to contact my team on 0161 856 4741."
He added, "Road closures will be maintained during the afternoon as investigations are underway, so avoid avoiding the area as much as possible."
Less than an hour later, GMP announced that two more children in the hospital are critical after two "unrelated" collisions in Heywood, another area of Greater Manchester near Rochdale.
"Last night (Friday, November 9, 2018), the police were called to two separate reports in the city," the troop said in a statement.
The first incident was called at 5:40 pm and the second one shortly thereafter at 8:15 pm.
The driver involved in the first incident stayed on-site to speak to the officers and no arrests were made.
In the second incident, officers arrested a 43-year-old man on suspicion of being drugged and causing serious injury through dangerous driving. He remains in custody.
The girl was taken to a hospital where she is also in a "critical condition".
Police Inspector Danny Byrne from GMP's Serious Collision Investigation Unit said, "Two girls were seriously injured after these collisions and we do everything we can to support their families during this difficult time.
"Two investigations were initiated and I would call any witness or anyone who provided information that could support our investigation to make contact."