I kept a diary up to the age of 12. I stopped writing it because an aunt I was living with read it. She pretended that she had "dreamt" me doing something i had written in my diary. I was furious, and felt really violated. My mother had just died, and the diary was the only outlet for my emotions. I retaliated by only writing obscene messages and comments about my aunt and how much I hated her after that. But if she hadn't abused my trust that way it would have been better for everyone. So please resist the temptation to pry. A real diary is like a special best friend you share your innermost secrets with, not a Facebook post.
I'm sorry you had this experience, publusher, however, I have to admit that if I had teenage children I would feel motivated to read their diaries in secret for no other reason than to reassure myself that they weren't boozing, taking, drugs, or having sex with their boyfriends.
If I could - I would compromise by taking their diaries to my doctor (or a trusted professional) - so that he or she could quickly thumb through the pages and tell me there's nothing I should worry about.
So... I don't read the diary, but I'm left knowing there's no disaster brewing.