Can’t help you much either, except that I’m sure I have read on this board before that some children/adults have weight problems on the dex and not on the cortef.  Maybe some other people can post their experiences on their treatment with dex.  Body image is problem enough for young girls with and without CAH so if you can swap meds and fix the problem then that would be great.  Sounds like you are doing all the right things anyway as far as food and exercise go (besides starving her!).  Good luck.