Teaching your kids to stop and simply notice their breathing is the beginning of a mindfulness practice that will benefit them throughout life.
And it's definitely not just for kids.
When we become fearful, stressed, anxious or angry, our brain releases chemicals that flood our system and send the signal that we need to go into survival mode (fight/flight/freeze). The more we experience this, the more automatic this response becomes, and we become wired this way.
When we focus on our breathing we interrupt the stress cycle and the brain stops releasing stress chemicals. We can literally rewire our brains, and as we practice this we become more likely to remain calm in stressful situations.
Our default response is the one we have practiced the most. It's never too late to rewire these neural pathways and change our default responses.