Breathing is a great way to regulate emotions and a very underutilized tool! Giving them specific breathing exercises, one at a time so as not to overwhelm them, gives them something to focus on and encourages habit-forming before they become upset.

I like to teach children about 'alphabet breathing" based on the book 'Alphabreaths' by Daniel Rechtschaffen. It takes each letter of the alphabet and gives the child a corresponding action related to the letter (i.e., "A" is for alligator: open your arms wide vertically whilst inhaling, then snap your arms closed like an alligator's jaws during the exhale, etc.). It's a wonderful book and highly loved in my household, I suggest picking it up for your in-home library!