So for an infant turning two mid travel if its an all BA itin then there is another booking set up to hold off the space on the return. The bookings is then TCP with the adult booking and seats are assigned as normal when traveling with children approx 72 hours before departure as children under 12 would be sat with an adult in your party.

Cant remember AA policy but i know some oneworld carriers arent as generous as BA and so if they are in the itinerary you may need to book as child for the whole journey.

My advice is call up and if it involves AA on the return ask the agent politely (if they dont already say they will) to contact the BA oneworld team and check the AA policy for infant turned two mid travel.