I took a friend's DD on a domestic flight. She was 11 and didn't have a passport or any ID. No problem. We vaguely look alike so people may have assumed she was related to me.

My kids are 11 & 14, and we have never brought or shown i.d. for a domestic flight.

No ID is required. You might bring copies of their passports and birth certificates just to ease your mind. Obviously photo ID is never required for kids flying domestically (not every kid has a passport), so the most you would ever need is a birth certificate anyway (to verify the age of a lap infant, for example).

We'll be flying domestically in a couple of days with our kids aged 3 and 5. I don't think we've ever flown within the US before and I've checked the airline website which says that no ID at all is required for children under 18. I just wanted to confirm that this is definitely true, including when flying out of DCA - I'd rather not bring the kids' passports and green cards if I don't have to because I'd rather not have to keep track of those valuable documents where we are going (but obviously will bring if necessary or if there is a possibility that ID would be required).