A página da TSA sobre identificação para voos declara :
Adult passengers (18 and over) are required to show a U.S. federal or state-issued photo ID in order to be allowed to go through the checkpoint and onto their flight.
Isso implicaria que os menores não precisam ter identidade.
No entanto, para confirmar isso, vou citar uma companhia aérea - página da JetBlue :
Para crianças:
Children
Children under the age of 18 and traveling on a domestic flight do not need to show identification or documentation unless they are under 14 days old (physician’s letter required) or are traveling as a lap child (proof of age may be required).
e desde que você não especificou se eles estavam viajando com um adulto:
Unaccompanied Minors
Unaccompanied minors must check in at the ticket counter. An unaccompanied minor does not need photo ID. However, the adult dropping them off and picking them up will need photo ID. This adult must be named in the reservation.