Maybe the selection of "under 2" results in them thinking lap/bassinet child (at reduced rate) so you cannot select a seat for that kid? This is likely a phone call situation, in my opinion.

You might also try to start by saying the kid is over 2 (say the 3-11 y/o drop down selection) which gets the kid a seat (likely also at a reduced rate) that you can pick during seat selection. I doubt your honest (e.g. the kids DOB makes them 2 or younger) answers to the passenger specific questions (passport name, DOB, FF#, passport number, etc.) would kick the kid out of the 3-11 y/o drop down selection and punt your total purchase (2 adults + 1) back to 2 adults and 2 y/o bassinet/in arms.