What could be simpler than deliberately breathing slowly in through your nose and then out, bringing attention to the sensation of how it feels. This purposeful action has a calming action on the brain. It’s not just for kids of course! For those interested in the Science, in it’s most basic form its the carbon dioxide that we exhale through our nose that creates the calming effect.
I have tried this many times with many children, just before they have had to sit school assessments. A technique to ground them and support them mentally. I usually used this in conjunction with another technique where children were asked to sit with the fingertips of both of their hands touching and both feet touching the floor.The kids really enjoyed the process.
Of course this technique can be used at anytime for any purpose. Never heard of these techniques? Give them a go (initially for a minute or two with eyes closed) and see what you think.