To help younger kids learn deep breathing skills:
I hand them a bottle of bubbles and coach them through breathing in deeply through their nose and then blowing out slowly through their mouth, just hard enough to release bubbles from the bubble wand. I ask them to imagine that the bubbles are all of their anger, sadness, and/or fear being released from inside their bodies. I allow them to keep the bubbles for their coping skills box and remind them to use the bubbles (and other items in the box) when they are feeling angry, sad, scared, etc.
The kids I have practiced this with so far have responded well to it. Of course, little kids love bubble blowing, but the hands-on activity really helps them to understand and practice deep breathing.