Children witnessing domestic violence

I have 2 daughters who visit their dad every fortnight. My ex and I divorced 5 + years ago, he has since repartnered and has a new son with his partner, he is also step dad to her daughter. Over the last 18 the relationship between my ex and his partner has gone down hill and is very volitile. My daughters have witnessed the arguments, destruction of property, verbal abuse and now physical violence, the perpetrator is not my ex but his partner! She screams and yells, hits him, pushes, scratches and punches him and deliberately smash/break things in the house, he yells back but for the most part tries to avoid or stop the fighting. I consider all this to be domestic violence and have zero tolerance for it. I have spoken with my ex and have heard excuses from 'its just a rough patch' to 'she will be gone soon shes just staying till i sell the house' and promises that it wont happen again. My daughters love their dad and want to see him but are sick of the fighting, they get scared and cry when it starts and want to come home. My ex knows how they feel, but wont let them come home when the fights start. The last visit there was a fight, it was so bad i reported some info to DCP. DCP spoke with my ex and case was closed, nothing they could do but have a talk with him. Because of what happened in the latest fight Im genuinely worried about sending my kids there! We do not have orders in place and up until a few weeks ago communication between my ex and myself was good (aslong as his partner didnt know as she doesnt like me) His partner has since blocked me from contacting him as she doesnt trust him or like me so i cannot contact him and now he will know Ive called DCP as the info could only have come from me. What do I do? He knows I wont send the kids back after the latest fight, of course he doesnt agree and thinks he will be having them as usual. I dont want my kids exposed to this anymore but they want to see their dad and seeing him without his partner will not fly with her so not an option. Help! What do I do???