You can sue a person who makes a false report to CPS if you can prove

that a false report was made to CPS and the person knew that the report was false or was made with reckless disregard of the truth or falsity of the report, AND any person who makes a report of child abuse or neglect known to be false or with reckless disregard of the truth or falsity of the report is liable for any damages caused. (CA Penal Code section 11172)