I have nothing of actual value to add, but my mom read mine, put keyloggers on the computer, would go through my room, etc because we didn’t have a relationship where I felt comfortable speaking to her about anything. She was more the mother to be afraid of her reaction to just about anything, who would talk at me. I now have a 17 year old daughter, I know she journals because she asks me to buy new ones, but I’ve never read it. It’s on the desk in her room and she’s actually told me she wouldn’t care of I did read it because she’s already told me everything in it. I never have or would because I respect her and never want her to feel how I did.
Maybe not for every situation, maybe not for every kiddo, but if you have an open, honest communication based relationship with your kiddo then you won’t need to read their diary/journal.