How do I stop a non-admin user from accessing System Preferences on my Mac?

You can set a passcode to block access to ScreenTime settings on your Mac.

Set, change, or remove a Screen Time passcode on Mac - Apple Support

Under the security pane of System Preferences / System Settings (depending on the macOS version of the device in question) there should be an option to lock system-wide preferences with your account password.

E.g., the att screenshot shows the option under System Preferences>Security and Privacy>Advanced... in Catalina (I'm not near my Sequoia or Ventura devices at the moment - but they have a similar option in System Settings).

Also, just to be sure, you set her up as a standard user, not an administrator? If she's an administrator, then she can unlock and set anything, so you will have to ensure she is only a standard user from your account.

daibrahim64 wrote:

I'm using Ventura and there is not the option "Require an administrator password to access system-wide preferences". My daughter is set up as a standard user.

It's there. Go to Settings > Privacy & Security , scroll all the way to the bottom of the panel, click > Advanced.

You have several options:

1 - if the Guest account can't change the settings on her iPhone delete her user account on the Mac and let her only use the Guest account.

2 - take her iPhone away and give her a simple phone that just makes calls and can text, i.e. a non-smart phone.

3 - and lastly enroll her in a computer science class at the local college.

Note: I'm leaning more towards #1. (I have to go to my grandkids [4 in college and 2 in HS] to figure out a lot of features on my iPhone).