From AA.com at
this link:
For transatlantic/transpacific travel or travel to/from Latin America, children under two years of age are required to have a ticket in their name for 10% of the applicable adult fare for the International portion. A 10% ticket does not entitle infants to their own seats. Contact Reservations for more details.
HOWEVER, I believe the flights the OP is talking about booking are on BA metal, but cheaper through the AA website; therefore, I do not know whether the above rules would apply. OP it sounds like you will need to cal AA to find out the particulars, which in hindsight would have been quicker than waiting for a definitive answer here. Please let us know how it works out.
Also if/since the fights are on BA metal, whatever the AA check-in counter does is a moot point as the OP would be checking in at the BA ticket counter for BA flights no matter with whom they were booked.