My boyfriend is getting a divorce and the child custody is becoming an issue, Need advice. Kids are 4 and 5. With bad mother and her boyfriend

My boyfriend says he was deemed an unfit parent so doesn't have custody. we found out today that the mother locks the kids in the bathroom and turns out the light, they also say that her boyfriend beats them and throws them in a corner for time out. He has threatened us (which we have recorded). The kids aren't happy and they say they don't want to live with them, She has had defax called 8 times by her own family but still has custody. She doesn't do anything with the kids and the kids put up a fight going back with he. What can we do? More info if needed