I.G.
I would suggest you try everything you can to break the habit. Trichtillomania can become a permanent habit through adulthood and the baldness that can cause great anguish to people who have it.
In kids it can be just a soothing habit (I did it as a child, but not to the point of hairloss) or it can be a sign of compulsive behavior - it wold be goo to consult a pediatrician.
If nothing else helps I would even consider getting her a very short, cute pixie cut for a while and let it grow back in once she is over it...
Good lcuk!