I wonder if anyone can offer any advice or experience on the following:

I recently booked a flight, via American but operated by Finnair, from ARN to LAX in economy for a lady and her 1 year old child. I booked a separate seat for the infant.

I see I can get a seat selection in Economy Comfort for around 110 euros per person - if I do that just for the mother, seeing that the infant has to sit next to the mother, what would happen? Would the infant automatically be allocated the Economy Comfort seat next to the mother?

Thanks for any insights.