Is the child a small adult when it comes to diving?
Definitely NOT.

Participation of children in scuba diving is growing every year. Instructors, dive masters, parents and pediatricians are all in growing need to understand the physiological differences between children and adults that relate specifically to scuba diving.

RESPIRATORY TRACT
Lung maturation and alveolar multiplication are progressive all along the growth period. It is very fast till the age of four, then gradually slows to stop at the age of eight. The elastic tissue is rare at birth, and then it increases gradually to reach complete development at 18 years.
These facts: relatively small lung capacity + less elasticity + scuba diving (breathing compressed air) = Possibility of air trapping even in a healthy lung.