from their website
Our goal is to have families seated together. We guarantee children 14 and under will be seated adjacent to an accompanying adult at no additional cost, including Basic Economy fares, if you meet all these conditions:
Everyone is booked in the same reservation
Adjacent seats are available in the same class of service your child is ticketed at the time of booking
You choose seats for the entire reservation or skip seats for the entire reservation (if you choose Basic Economy, then skip choosing seats for the entire reservation)
You don’t make a change to your seat assignments once they’re assigned to you
The plane’s seat layout allows it based on the number of children in your reservation
The original flight isn’t switched to a smaller aircraft
If these conditions aren’t met, we will still try to seat children adjacent to an accompanying adult.
https://www.aa.com/i18n/customer-service/support/customer-service-plan.jsp
