Would need to know more about the situation and how the assault has been managed, thus far.
Report to authority and counseling
Please report this to the police IMMEDIATELY. The longer you wait, the more damage will be done to your granddaughter.
Hopefully someone can make her feel safe by removing her from the environment. Please explain further. And if she has been removed. Just love her and let her be a child.
Has she been removed from her father's custody?