Sometimes younger students may act out when they feel dysregulated and don't know what to do with those big feelings. However, mid-meltdown is NOT the time to introduce breathing exercises. They won't hear you and soon you may feel yourself spiraling to regain control.
Teaching breathing exercises during calm, regulated times will teach students that it is normal to feel this way and what they can do about it. Teaching students breathing techniques opens dialogue to help them to be able to give words to those big feelings and how it makes them feel inside. 
The five finger breathing technique shown in the picture above is so beneficial for students to learn because it can be done anywhere at anytime and no one has to know that they need a moment to calm themselves down. They can put their hands in their lap and breathe away without drawing attention to the situation.
There are many types of breathing exercises and they aren't just for children! 

High levels of success are dependent on staying focused on effort, instead of results.  Coach John Wooden, UCLA basketball coaching legend who led his team to multiple championships became an expert on winning in his lifetime, developed an inspiring outlook on failing. He is quoted as saying, "I had mistakes, but no failures. We may not have won a championship every year and of course lost games, but we had no failures. You never fail if you did the best you were capable and gave full effort." 
Teaching children how grow through challenges can be challenging, but Wooden's example is an excellent starting place.
When taking a loss or experiencing defeat, teach them to ask themselves:
1. How much effort did I put into preparing myself? For this test, this skill, this sporting event, whatever it is they are working on - on a scale of 1-10, how'd I prepare?
2. What is one thing I could improve on in my preparation?
Focusing on what could be done differently is much more effective use of time than beating yourself up over what went wrong.
Daily focus on effort is a skill that must be taught and as we age can truly only be measured by ourselves. Considered to be one of the greatest NFL linebackers in the history of the game, Ray Lewis said, "Effort is between you and you."