One method of explaining the respiratory system to children is to break down the system into its component parts. Explain the major organs of respiration, such as the lungs and the diaphragm. Since breathing is an obvious physical change, have children take deep breaths to feel their lungs expand.
Explain that the diaphragm, like a rubber band attached to balloons, stretches and flattens downward to expand the size of the lungs. This is what allows respiration to happen. Then, explain why respiration happens, and what goes on inside the lungs during the process. The alveoli, at the end of bronchioles, allow oxygen from the air breathed in to hitch a ride with blood vessels, which then leave behind carbon dioxide to be exhaled.