Wow, tough crowd here.

I feel bad for the OP. While more research/clarification beforehand may have helped the situation, it's also fair to assume that at the first point of travel, the first airport that you check in and fly from that airport to your connecting airport, onwards to international travel, that they wouldn't require a passport, as it's strictly domestic travel.

All this could have been easily avoided if TA/GA's at DCA/IAD would have sent ANC the passport information. It's not that hard, especially considering it's a minor. A note could have been added to the PNR, asking that the actual passport be checked at ORD/WAS. If you asked about the UM fees, etc, over the phone, wouldn't the res agent have noticed there was a connection and explained the UM policy of no connections? There could have been more done to salvage this situation, and it's unfortunate that UA did the bare minimum, and even so, screwed over the OP. I must decree, UA dropped the ball here.

I believe compensation is due. Then again, that's my opinion, and the airline may disagree. But, tough crowd, indeed.