Document everything. Keep a journal, write it all down, date it all. Write down all conversations you have with your daughter regarding the care of your granddaughter especially when she admits to neglect and what would prove her to be unfit.
And you keep calling CPS/DCYS when they (your daughter and her boyfriend) put your granddaughter in danger and neglect her. Or you call the police, who are mandated reporters. Call as many times as it takes so that it's officially on file each and every single time. If she comes to you filthy and unfed and with food poisoning, call CPS to have them come see the state she's in when your daughter drops her off you pick her up. Take photos and video. Print the photos out and make sure the date is on them.
Just keep doing it for as long as it takes. Encourage other family members to do the same when they have the knowledge to do so.
I'm so sorry that you're in this position. I hope you don't mind my prayers for your daughter and granddaughter.