I would definately do it. It sounds like she needs for it to be shorter just for hygeinic reasons if not so she doesn't wreak it even more. Maybe when she's older she can have it longer but if she won't let you help her with it, make it managable for her now. I just cut my DD's hair shorter to her shoulders, while she's only 3, it is so much more managable for me to comb and wash and put up for her.