In my experience I think that you would be able to. I recently booked my family of 5 a set of tickets on ba.com using avios. For some reason, the web site would not let me buy a seat for my 8 month old (even in the same itinerary as the rest of us) - it just automatically assumed he was a a lap child. I went in and entered his age as being over 2 (which would require his own seat) and went ahead with the booking.
After booking, I called AA and had them change his age to the correct age - no problems whatsoever.
I would assume you could do the same with your UM's. Just enter an acceptable age and then call up AA to have them put the correct age in and pay the appropriate UM fees.