Oh Brookly I am so sorry. I know how hard this is.
I am going through this myself. My H came to visit the kids, hasnt' seen them since Nov and brought his OW and her 2 kids and they all spent a great family vaca together....
I tried to speak to H about how this isn't good for the kids they have been throught a lot this year blah blah blah. He heard none of it and of course did whatever he wanted to. He told the kids she was his friend. I am not sure what else he said but they really didnt' say much about her to me.
I read that your kids look to you to see how to handle things and I have tried my best not to say anything about about their dad and just act like everything is fine. I tell them it is ok to be sad and mad at Dad but that he loves them. I told S8 last night it was ok to miss Dad that sometimes I miss him also. But for the most part I try to act like everything is ok so they are ok too. I know that is hard. It was really smart to call your parents for help then. We are all here for you.
You have to keep telling yourself it is your H's loss. You are an incredible woman and he is going to be the one missing out.