It's so hard when kids are involved, but there's just no way I would allow another woman to move in! NEVER! All I have going for me right now is that my husband hasn't actually started a new relationship with another woman (yet!). It's the only thing that gives me hope in such a miserable situation. I can only imagine the pain you are feeling right now, and at the same time, prepare myself for the same.
How does your husband think things will work with another woman in the house? what would the vibe be? would you be expected to fake it in front of the kids that you are one big happy family? how old are your kids?
It could help to do as I did and print out all the relevant responses from EXPERIENCED poly people and show them to him, it made my husband to realize (somewhat) that his head was in the clouds...
My thoughts are with you...keep strong (and busy!)