Reporting back, I had 2 other passengers on the same PNR as me with the infant. I got the Priority with the carry on for free since I had the infant. The others on the boarding pass said underseat bag only. There was no option to upgrade the other 2 when booking the flight since my ticket with the infant was priority with carry on already for free. The only way I could see was to pay for the seat upgrade, priority, carry on combo but you couldn't do the priority and carry on by itself. I was boarding group 1 and the other 2 people were group 2 but they let us board together in Group 1. I'm not sure if they were checking bags or not for the other 2 people so you might get away with it. YMMV.