Compared to adult anatomy, children have a floppier epiglottis and vocal cords that slant upward. They also have a funnel-shaped larynx, which can be difficult to navigate around, especially due to the narrowing of the cricoid cartilage.
Those factors make using scaled-down versions of adult endotracheal tubes (ETTs) on children risky because doing so can result in airway damage, oxygen deprivation, and ventilation complications.