Citizen's arrest only exists if there's reasonable suspicion a proper criminal event has occurred, and conveys no liability protection: if the person "citizen a

Citizen's arrest only exists if there's reasonable suspicion a proper criminal event has occurred, and conveys no liability protection: if the person "citizen arrested" is found to have not committed a crime, the "arrestor" can be civilly sued for huge sums of money, and potentially criminally charged, themselves (though this is, I understand, uncommon). In general I can't think of a way private interrogations are legal in the US, though the airline will likely claim that the kid was "always free to leave" and get away with this.