Blankeney was with siblings and adults heading to the Los Angeles Police Department's 77th Street Division for a party.
"Somebody had her by the hand crossing the street when she got hit," said the girl's grandmother, Krystal Roberts.
The car missed Blankeney's mother and 14-year-old brother, Jayson, but struck her and her aunt.
Witnesses said the car kept on going.
"It happened so fast," Roberts said. "One minute they were walking across the street, the next minute he looks up and Chay Chay is up in the air, and that was all he could remember."
Marshay was in convulsions. Her brother tried running after the driver.
"He just ran after the car," Roberts said. "All he could think about was his baby sister laying there on the ground and that she had been hit."
The toddler, who was dressed as Alice from "Alice in Wonderland," was taken to the hospital in critical condition. She is expected to survive.
Her aunt was also taken to a hospital and has since been released.
The car is described as a compact car, possibly a Toyota or a Pontiac, with front-end damage. Witnesses said the car was grey, white or red. It had tinted windows and was an older model.
"We have no animosity toward him," Roberts said. "We just want him to own up to what he's done.'
Anyone with information is urged to call LAPD detectives at (323) 290-6063.