The accident took place at the Argyle Street roundabout in Boston at around 3.45pm on Friday as the girl was walking home from school with her brother and friends.
A spokesman for Lincolnshire Police said: “An eight-year-old girl was in collision with a black car in Arglye Street around 5.30pm on Friday 3rd December.
“The vehicle did not stop at the scene and the girl sustained serious injuries.
“We are keen to trace the driver to speak with them as part of our investigations, and would ask anyone with information to come forward. We would also like to see any dashcam footage which may have captured the incident.”
The child was taken to Pilgrim Hospital for treatment, where the fracture was confirmed and her leg was put in a cast.
The girl’s mother, Dimitrina Fileva, has spoken out to thank the young people who stopped to help her daughter and waited with her until an ambulance arrived.
Speaking to the Standard, she said: “Those kids were amazing. I’m sure it was a shock for them as well.”
She also took to social media to praise the young people, writing “Fortunately a few kids stayed with her waiting for me while she was crying on the pavement. Huge thanks to them and their parents for raising such sensible little people. There were two teenage boys as well, thanks for being such great role models. Your parents should be so proud.”
The cause of the collision is not yet known.
The police spokesman added: “At this stage we are keeping a completely open mind about the circumstances.”
Any witnesses to the collision, or those with dash-cam footage, are asked to contact police by dialling 101 and quoting incident 331 of December 3. Alternatively, email [email protected] quoting incident 331 of December 3 in the subject line.