Dating/Cohabitation while separated

My husband and I have been separated since the end of March. We have a 3 year old and an 18 month old. He visits them at my home every Tuesday and Thursday night when he gets off of work, and takes them overnight every Saturday at his apartment. He has been dating a woman who lives across the country for several months now. Today he informed me that sometime in the near future (before Christmas) she is going to be moving here with her toddler and they are will be moving in together. I told him that was fine and I had already considered the possibility of this happening, but expressed concern about the impact of this living situation on our children. The more I think about the situation the less comfortable with it I am. We do not have any separation agreements. Is there anyway that I can legally keep him from cohabiting with his girlfriend before the divorce is finalized, or subsequently prohibit him from taking the children to his home if he does move in with her before the divorce? I have no desire to try to limit what he does in relationships after the divorce, but I am not comfortable with the children staying at his home while he is cohabiting with his girlfriend and her son while we are still legally married.