I’m in the decison stage also with letrozole.  My ten year old son has begun puberty and it is my understanding that they should only begin it if they are in puberty....maybe it is different for girls.  I do know that letrozole supresses  estrogen production so at the tail end of puberty the growth plates won’t fuse because it takes estrogen for this to happen.  I have heard both sides of the fertility issue and believe that they don’t have a lot of hard data to prove that it does.  In fact, our endo believes the opposite to be true.  If my son were to take letrozole his puberty would progress as his body wishes but we have to monitor bone age and get blood draws more regularly to determine whether he is adequately suppressed.  I guess I would wonder why they want to put your daughter on it if she hasn’t entered puberty.  I also wonder why they would lessen her cortef if she is well surpressed...I thought that is what we want....them to grow but remain surpressed.  Good luck with you decision.