These kids just use someone elses phone or computer to access what they want or to erase content. Her psychologist should be able to help in this situation, and bring you peace of mind.

 
Perhaps this will help, if you have her Apple ID password.
 
How to Reset Forgotten Apple Notes Password on iPhone, iPad, and Mac

^^^^^^

I looked at that too, but here’s the problem… Resetting the Notes password will simply allow the person to use Notes again, it would still not give them access to the old notes that were protected under a different password.

Apple makes it really clear that there is zero chance a person will be able to access password protected notes without the exact password used to protect those particular notes.

After you reset your password, you won’t regain access to your old notes, but it will let you lock notes going forward. If at any point you remember the old password, you can go to the old note and enter the old password to unlock it.