My granddaughters mom does drugs. What can we do to get them?

Contact social services or DCS in your area and report her. Unless and until there is proof of drug use and abuse or neglect, there is little you can do.

How do you know this? Where is the father?

As noted, your only option is to call Family Services. When you do, let them know you are willing to foster the child. But there is no guarantee they will place the child with you.