It has been shown in many health studies that there are many physiological benefits to breathing into the belly. These benefits can include the slowing down of your heart rate and getting more oxygen to the lower lungs, the blood, and effectively your whole body.
It is often a natural process for children to breathe into their belly. With this practice you can help them enhance and retain this skill in a few ways.
One of the best ways to do this is to ask them to breathe into their belly like a balloon: