Setting: Earth, 2007. While some characters have healing magic, they are feared and will not be reachable for this incident.
My character is a thin 8 year girl. I've always pictured her as ending up using forearm crutches (if only because they allow for free hand/s unlike a cane) as the result of something traumatic rather than a problem such as hip dysplasia that started at birth, but I'm willing to go with that if nothing else sounds reasonable. I've looked through other tags here, Googled reasons for forearm crutches, and researched leg injuries in children.
Does it make sense that she, riding in the front seat of a small car next to her mother (who intends to kill both of them), ends up with a broken hip, thigh and/or pelvis from the vehicle rolling or crashing into something at high speend? I believe at that point, she'd have the bones reconstructed, but require more operations as she ages and those bones grow and spread thanks to puberty.