If a car hit your child, remove them to a safe location. While it is generally preferable not to move them for fear of inflicting additional pain or injury, you do not want to risk a second impact. Do not move your child if they are having difficulty breathing, you see signs of potentially broken bones, or if they are unconscious.
Contact the police immediately. Ask someone nearby to help you obtain information from the driver who struck your child, nearby witnesses, and take photographs of the accident scene. This information may be helpful later.