L.H.
Dermatologists are the physisicians who deal with hair loss. It will be difficult to find one that deals with child hair loss. I totally agree that you need to look into this.
Here are some picts of different types of hair loss, maybe that can point you in the right direction.
http://www.hairlosstalk.com/hair-loss-children/childrens-...
Other things not addressed here are thyroid imbalance, vitamin dificiencies, and autoimmune disorders.
Again, a dermatologist is trained to find the causes of hair loss.
Best wishes.