K.,
My son had to wear a helmet as well. You would be surprised how common this seems to be getting. The previous response has a point with being sweaty when wearing the helmet. One other thing to consider is if there is a spot on the helmet may be irritating her. If you are not already aware of this, when removing the helmet, check for red spots on the points of her head where the spots are NOT suppose to be--if they are there, then it may be pressing in the wrong spot. Also, if the red spots do not disappear after an hour, then it may not be fitted right and your daughter may need to consult the orthopedist--which she should do anyway regarding this issue. Ensuring that your granddaughter has tummy time, sitting up time, any time off her back is really important in the reshaping process. My son had plageocephaly, misformation on the sides rather than the back of the head. This was due to tight neck muscles so we were referred to a physical therapist for him to strengthen both sides of his neck so he could turn both ways. I would ask if this may be necessary as well.
Sorry, a little more advice than you probably asked for--I just know how curious we were when we went through this.