Immune system is immature which increases susceptibility to infection. Intercostal muscles aren't developed until school age. Any compromise to diaphragm can cause difficulty. Ventilatory space is small and easily compromised by abdominal distention which can cause elevation of the diaphragm. Thin chest wall.
Any pressure interferes with respirations.
Breath sounds can be heard well. Cartilage of ribs and sternum are softer and more flexible than adult. This, along with a thin chest wall and poorly developed intercostal muscles can cause retractions. In general, the younger the child, the smaller the volume of air space and the more rapid the respirations must be to obtain adequate oxygen for the body to meet it's needs. Count respiration for one full minute because often it is irregular.