Okay, now that it is all done, let me review.
1. You cannot book kids under 12 on BA website. I just went ahead and put wrong DOBs al m'nas to change it later. I was not trying to wiggle out of the UM fee since the UM fee goes to the carrier anyway.
2. I then called USAirways, who happened to be the carrier, and added the UM fee. I did not bother correcting the DOBs at that point since I felt it made no difference anymore.
3. Data point... it took a loooooong time for the reservations to ticket. Basically I had 3 reservation codes on each passenger. 1) a BA reservation code, 2) A USAirways reservation code with a BA ticket # but no USAirways ticket # (hence could not choose seats) and 3) a USAirways reservation code with no ticket #s at all but with seat assignments. Weird. Maybe that is typical for BA to USAirways but it worried me so I stopped into a lounge and had one of the workers there fix it for me. They consolidated it all for me and it was fine in the end. Maybe it would have been fine on its own without me asking for them to help me, but I didn't want to take chances.