If a child with trach had an open enough airway to breathe past their short trach, wouldn't most of their exhale go thru their uppr airway, not trach?

Could be, but...: The trach also shortens the length of passage the air has to travel, which decreases the work of breathing, giving your child more calories to use to grow.

Not always: The trach is the path of least resistance and people with good upper airways will often use the trach pathway. Also, the diameter of the trach takes up part, or a lot of the airway and can force the person not to breath through the upper airway.