Another vote for CPS. I lived in an abusive household. Forty years later, I still harbor anger for everybody who suspected something was going on but did nothing because they didn't want to rock the boat. I can't believe that all those neighbors heard what was going on but never said a word. I can't believe my aunt saw what was going on and just watched as my caretaker beat me up (she also watched her own son beat up his siblings, her own children and did nothing about that either). What you've witnessed barely scratches the surface of what those children are subjected too. You have no idea what's happening behind closed doors. You also have no idea what she could have said to the children to keep them quiet.
I know you are very afraid of losing touch with those kids, but from a purely monetary perspective, it is going to be way cheaper for CPS to place them with family members rather than a group home/foster care.