I read my child's private diary that was sitting on her desk
Why did you read her diary?
She needs to learn not to leave her diary out.
You shouldn't read your child's private diary. They may leave their diary out because they think they can trust their parents. If you read their diary, they may feel like you betrayed their trust.
I don't care. If she needs to learn not to leave her stuff around then it's a lesson she needs to learn.
You can just tell her to not leave it out. Do not go through your child's things, they will not trust you.