Grief due to divorce is different from grief due to death. The other person is still around after the loss. When the other person shares kids with you, you have to keep on dealing with them after the loss. Interacting with them is a recurring reminder of the divorce and the differences leading up to the divorce.
All of this is on top of (or underlying) real differences of opinion or preference about issues concerning the children. So you're going to be mad. And you won't be able to hide that from the kids. Nor will you succeed in making rules (like the one reported above) that keep the anger out of the house.
What you can do is admit you are mad, admit that you are mad at your ex, but you are also mad at the situation, which stinks. And then add that it's not the kids' fault, not any one person’s fault altogether, and that it's not fair to them and you're sorry they have to go through this. Then you can all work through the messy feelings honestly, though painfully.