Call the mother (during the day) and try for written permission for her to live with you. You don't say where her biological father is.
If the parents don't call the authorities, it is unlikely anyone else would.
You can consider a call to CPS but that runs the risk that the child would be placed somewhere else. If her parents make her return home, do make the CPS call and make it crystal clear you want her placed with you.
This answer DOES NOT establish an attorney-client relationship. This answer is based on the limited information provided and is not intended to be conclusive advice. There are likely other factors that might influence or change the advice after a more lengthy consultation.
You have to be careful. If you don't tell her parents that she's with you, you could be charged with harboring a runaway. I suggest having her call them to say that she's safe and will talk to them tomorrow. Do try to get permission to keep her if you can. Unfortunately she may be the only one working in the household. Mom may claim to police that you are holding her against mothers will. Be up front with police. Consider that call to cps if there's been abuse.its an obligation of law..
I am not your attorney until you hire me and pay a retainer.
Sign up to receive a 3-part series of useful information and advice about child custody law.