This has always frustrated me. The best workaround I've figured out is remembering the AAdvantage numbers of my family. After clearing out the pre-populated passenger info, which AA just draws from in the order you have it in your preferences, I can put the first, last name and AAdvantage number, then it will fill in the rest on its own. I've had issues in the past with the country and state not filling in, but lately it's worked fine for my kids. (But my kids also recently got real IDs, so maybe it is tied to that?)