I have done this on many occasions. Put the flights you want for the adult(s) on a 24 hour hold. Call into AA to buy the child's ticket, referencing the PNR(s) for the flights on hold (they will be linked). This works for award tickets too.

Keep in mind that you will be required to check in at the airport for the child (in order to verify the adult is also flying). Checking in through the app or a kiosk will not be possible. So get there a bit early.