Asianae Thomas was at a family gathering when she walked off the curb in the 2400 block of North Mountain View Avenue on Sunday.
Family members said they followed the truck involved, but no arrests have been made. San Bernardino police said they've received conflicting information about the vehicle.
Thomas' grandmother Charlotte Matheny urged the driver to turn himself in.
"I'm asking you, whoever you are, to please turn yourself in," Matheny said. "Parents, tell your son. Tell him what he did to my grandbaby."
Anyone with information is asked to call Detective Peck at (909) 384-5664.