Thanks all - I decided to just check him in at the airport. Curbside check-in couldn't do it; the curbside agent showed me his computer screen indicating that I needed to see a CSR rep to do check-in because of the upgrade instrument being used (even though it was UDU). So I started to wait in the 1P line for check-in when I thought to try a kiosk. I used my 1P card to log in, then indicated that I wanted to check in someone else for a flight, and then entered my baby's MP#, which (finally) allowed me to check him in for the flight and print the BP. Phew!

But still, quite annoying...