I had the very same problem a couple of times spread over 3 kids. Easiest way is to do the adults online to get the flysmart discount. Then call up the M&M hotline or regional sales center and have the infant added... I remember once that we added the infant upon checkin, as I simply forgot to do the infant reservation...

Just as a cautious note. The infant must be still infant (<2yrs) on date of return flight, otherwise you get into trouble. The reservation system doesn't prevent you from booking an infant ticket, where the infant is older than 2 years upon return.

I learned this the hard way and had quite some discussion with LH checkin and sales staff at ORD. I guess there is some safety rule out there, where passengers over > 2 years must have an assigned seat...

Fortunately the flight wasnt full and eventually the LH station manager as a sign of goodwill, customer service, gave us an empty eco seat for our boy free of charge (the full fare Y are ridicilous, dont remember exactly but would have been in the 2k USD range for a single ORD-FRA)...

Marc