The schoolgirl, named as Amelia in messages left with the flowers, was knocked down at around 4.45pm on Monday.
She walking on a grass verge along Bullerthorpe Lane in Colton, when she was struck by a blue Ford Focus travelling in the same direction.
She was taken to hospital and died yesterday as a result of her serious injuries.
One message read: “Amelia, I can’t believe this has happened. You were one of the nicest people I’ve ever had the pleasure of meeting. Sweet dreams, love from Jack”
Another said: “Amelia, Taken too soon. You will be missed and forever in our thoughts. Sleep tight, sweet angel. Lots of love and prayers, Maggie, Mick and Vicky”
Police are appealing for witnesses to the incident.
Sergeant Mat Tunney, of the major collision enquiry team, said “We are appealing to anyone who may have witnessed this incident, who did not provide their details to officers at the scene, to contact us.
“We are also trying to contact the driver of a vehicle that is thought to have been directly behind the Blue Ford Focus when the collision occurred and may have turned their vehicle around and left the scene prior to emergency services attending.”
Anyone with any information is asked to contact PC Tunney on 01924 293047 quoting log number 1042 of 30/05.