I have a valid state ID card, but my kid's passport expired and we are actually flying to a city where a consulate/embassy exists to collect his new passport.
Can my kid fly with an expired passport domestically within USA?
Children traveling domestically by air do not need to present a valid passport at the airport. The specific rules of travel vary slightly from airline to airline, and many require that if the child is traveling with an adult, the adult must provide documentation to prove his age. Before booking a domestic flight, check with the airline about its specific requirements for children so that you're prepared with the correct documents, if needed.
But I'm unsure if the information is trustworthy or relevant to US domestic travel.