LJ29, you are right to be upset by this. My STBexH left me in Sept for his OW and he is currently on a luxurious holiday in Turkey with my boys, his OW and her DS, and I completely hate it.
I don't have the amicable relationship with my ex that you have with yours, we don't really speak, just text and email apart from a few words at drop offs and pick ups. It tears my heart out that they now have this unsuitable (to put it politely, three times divorced, no problem having an affair with a married man) woman influencing their lives. He has now bought a house with her and my boys spend every other long weekend with them both.
I hate it, but I have no moral right to object. My boys need to have a relationship with their dad, whatever my feeling about him are, and unfortunately this includes the OW. So, for the good of my DC, I suck it up. I don't have to like it and MN is a good place to rant about it. Your ex is moving on and it's good that he wants to include his DC in his new life. I would be much worse for them if he didn't.
My feelings are that when they are with my ex, he's their dad and he makes the decisions on parenting them. They are with me for the majority of the time and I make most of the parenting decisions. We only really discuss major issues, like what school they'll go to or if one is quite ill, or where they'll be if one of us takes them abroad. Sleepovers or holidays in this country are none of his or my business.
I can see that with your more amicable relationship you have been used to discussing more than this, but as he gets more into this relationship and possibly they move in together, you are going to have to suck it up. 