Incest Survivors Support Group
Incest refers to any sexual activity between closely related persons that is illegal or socially taboo. Consensual adult incest is very rare. Incest between adults and prepubescent or adolescent children is a form of child sexual abuse that has been shown to be one of the most extreme forms of childhood trauma. If you or a loved one is a survivor of incest, join the group...
Also if she is more comfortable with writing to you then maybe for now at least, thats how you should communicate with her. She may be feeling shame or guilt or any number of emotions that at her age are overpowering and difficult for her to express.
I would keep the journal a secret between the 2 of you so no one (her father) will look for it to use it in any kind of negative way. Maybe one with a lock on it and a special place to hide it?
I wish the best for you and your little one and I hope you get to the bottom of this soon.