(((Cinders))) I can relate to how you are feeling. My kids have had to go with my XH and OW (who is now his W) since my D was a newborn and my S was 2. Over the years, I tried everything I could to stop the OW from being a big part of my kid's lives...I asked that she not be in the car when my XH picked up the kids or I asked that she not come to the sporting events. I think this OW was more of a monster than most because if I asked her not to do something, that gave her more fuel to want to do it. You would think she would have tried to accomodate me since she helped destroy my family but she continues to this day to do everything she can to try to make me miserable...I believe it all boils down to her guilt.
It is really unlikely that there is much that you can do to keep the OW and her family from being a part of your children's lives as long as she is part of your H's life.
If I had the strength years ago, I would have followed the advice of "keep your friends close and your enemies closer". I am not saying that you should be friends with the OW but you may need to do your best to act "as if" do to keep your children out of the middle. My children have been stuck in the middle for all these years and if there was some way I could go back and undo it, I would. I wasn't able to keep my children's family together for them but I should have been a bigger person and figured out a way to keep them out of the middle.
Sorry that probably isn't what you want to hear...I just know how hard all of this can be on the completely innocent kids.
