I do not know how the CPS works where you live.
I think you can risk that everyone turns against you – but it is the right thing to do. Abused kids can be very loyal towards their parent but I think in the long run it is better for them to meet an adult that takes a stand and says this is out of line and has to stop.
I do understand that mental illness can be very difficult but we should never allow that a child is attack by its parent – I think I would intervene.