Observing the breath can be helpful for a child if they are angry, tense, or anxious. We have found that 5-finger breathing (or pawsing for breath if you are a Moffle) is a great way to help a child slow down & become more grounded & present in the moment. Ask them to calmly trace the fingers of one hand with the other hand, breathing in as they go up & out as they go down.
Encourage your child to let their breath come naturally as they do this. There is no need to try & control the speed of the breathing, or to breathe any differently to normal. Reassure them that breathing happens all by itself & we are just going to take a few breaths to notice it.