As mentioned, a flight on AA might arrive at a gate at Terminal A,B,C or D (and change at the last minute)......so it's more of a sure thing to meet grandparents at United at Terminal E. Wherever you land on AA, take the SkyTraaaaain (staying inside security) to Terminal E (daughter should enjoy the views) to meet grandparents.

Terminal E tends to be less busy, less chaotic than the AA terminals, so should be a calmer hand-off and easier to find one another. Maybe the UA ticket agent at Terminal E would give grandparents a pass to get through security to meet the child?

Map of DFW Terminal E at
http://www.dfwairport.com/terminals/e.php