To help kids practice mindful breathing you can encourage them to imagine the belly inflating and deflating like a balloon as they breathe.
Bringing a visualisation to the breath can help them to focus their attention in the breath. They can even place a hand on the belly to feel the gentle rise and fall of the breath. Remember not to worry if the mind wanders, thinking is all part of the process. Each time you notice the mind has wandered off, gently bring all of your attention back to the rise and fall of the belly.