Did your mother ever ask you to count to three when you are upset? Or before you spout off at someone in a way you will later regret. Turns out that’s not bad advice. 
Mine often added, “Take a deep breath.”

Yes, breathing is one of the best ways to calm down. We’ve become a little more sophisticated than that these days. Breathing exercises abound, and the benefits are clear. 

I know one helpful breathing exercise. It is called 4-7-8. Inhale to the count of 4. Hold to the count of 7. Exhale to the count of 8. After the first one, you inhale more deeply, which is soothing and calming. You will feel the results in less than a minute.

You can share this with children to help them feel calmer. Some schools are implementing this, along with yoga and meditation.

Another helpful practice is to pretend that you are blowing on a candle. Imagining that you are breathing in the scent of a candle through your nose and exhaling through your mouth. Very centering.

Last, I would like to share a recipe for a Calm Down Jar. Children not only like to create these but find it helpful to observe, especially when agitated. 

Add very warm water until the jar is half full. For children, make sure the jar is plastic. A water bottle will suffice. 
Add clear Elmer’s glue and stir until it is combined with the water. (The more glue, the slower the glitter moves) Fill until you are close to the top.
Add 1 or 2 drops of food coloring. 
Shake the contents and wait until the bubbles subside.
Tap in small amounts of glitter. (Extra fine works well) Adding several colors is attractive. (Try to include silver if available)
Shake well. 
Let it rest until cool. 
Close the lid tightly.
Enjoy.

Not only is it beautiful, but it is a great stress reliever.