I need to book 2 adults + 1 child (4 yrs) + 1 infant in lap. I can't do this through the United.com site.

a. Should I bite the bullet and call the 1.800 # for ticketing this and pay the 5$x 4pax = 20$ fee?
b. Should I book the 2 adults + 1 child on United.com and call 1.800 to notify that the child is 4 yrs and there's an infant as well?
c. The 2 adults and 1 child each have their respective MP #s. If I book through the 1.800#, presumably we can add the MP# into the record for Easy Checkin online and MPD425. Correct?
d. Can i wriggle out of paying the 20$ fee by saying united.com doesn't support booking child fares or infant fares?

This is for a domestic trip, not an international routing.