I have always taken my childrens passports to use as ID, even when flying domestically, however, we've never been asked to show them.

On our last flight, to Hawaii, I brought all 3 of the kids passports. However, my 15 year old son's passport recently expired, and to avoid any potential problems, I did bring his birth certificate with embossed seal as well. Never needed any of their IDs and was told, in fact, that we didn't need them.

Flew this week to MSP with my 10 year old daughter and was never asked to show any ID on her.