Is she hungry all the time or just when going through a growth spurt? If she is going through a growth spurt, she may not be gaining weight but growing in height. Also, has she been checked for diabetes or cystic fibrosis related diabetes? If there is a diabetes issue this could explain always being hungry but not gaining weight, at least it is one of the symptoms. If her HA1C looks good, ask about getting a meter to spot check her in the am before she eats or drinks anything and then a couple of random checks during the day. This is a much better way of diagnosing diabetes in a CFer and is more accurate. I know anything over 110 in the am pre-meal is to high and anything over 150 on a random is too high.
Since more of us experience a lack of appetite, I would really check the diabetes thing to rule it out at least.
Keep us posted,