Except when they do, and it's not departure airport dependent. It's "are you flying to a destination that doesn't heavily fine for delivering undocumented travelers, or a country that doesn't require a visa, and have you used that passport before and is it still valid" dependent.

Honestly, who has not had their passport checked at the gate or heard the announcement "if you don't have 'DOCS OK' stamped on your boarding pass, approach the podium..."

Hard to fault UA here. Parents had the responsibility to know the rules. If I'm betting on this, the kid had bags to check. No way in hades could that bag be checked without a passport in hand.