I know it's wrong

My ex and I have two young children together he was with this girl let's call her jess and we got along great me and jess even became friends and it was just lovely that we all got along and spent time together my ex suddenly left jess for another girl. This girl is a drug abuser I still stay in contact with jess and let her see my children as she loves them like their her own but I refuse to let this new girl be around my kids and I have actually completely stopped letting him take the children near this other woman. I just don't know what to do I know I shouldn't keep them from him but I'm scared of my children being around drugs or something what would you do?!