The glorious idea I copied is the Take A Break Box. When E1 or E2 is having a tough time, whining too much, getting frustrated, being sassy, etc I simply say, "You need a break."
This is their cue to go take some time to themselves. This sentence, "You need a break." is not the starting point for negotiations. It is not the starting point for a conversation of what behavior needs to stop. The sentence is a statement which needs no response. If I have said, "You need to take a break." E1 has learned that really means, "Get the hell away from me before I lose my shit because you are acting like a jackass."
Side note: At what age can I actually say that to my kids?
My friend has a Take A Break Box for each child in her family. I only made one (for now). In the Take A Break Box a parent should place items that are quiet and will refocus, recenter, and calm the child. Every child is different so the Take A Break Box items will differ. Ideas for your child's box: play-dough, silly-putty, coloring books, books, legos, paper and pencils, flashcards, anything independent that will allow the child to calm down and help them change their behavior.
Our Take A Break Box has crayons, stickers, construction paper, books on both their reading levels, Super Mind for E1 and Mighty Mind for E2.
So far this new behavior plan is working. It provides an outlet for the girls and gives me time to gather my thoughts before talking to my child about the situation that led to taking a break.