password protect my text messages

Currently, there is no way to lock specific apps using a password, however there are two ways you can prevent them from being accessed.

The first method uses Guided Access which prevents you from closing the current app.

1. Open settings and go to Accessibility
2. Scroll to the bottom and click Guided access
3. Turn it on and add a passcode

Now if you want to lock your daughter into a game so she can't use any other app

1. Open the app and triple click the home button (iPhone 8 and older + SE) or the side sleep button (iPhone X and newer)
2. You can either click "Start" in the top right corner to just restrict her from leaving the app, or you can draw shapes and add areas where touch will not work.

The second method is using Screen Time to make apps restricted

1. Open settings and scroll to Screen Time
2. Turn on screen time
3. Click "This is my child's device" so you can set limits
4. Click on app limits and set a limit to 1 min, then you will have to enter the passcode every 15 minutes when you are using messages, but your daughter will not be able to open it.

You can use Guided Access to restrict here to one app.

Use Guided Access with iPhone, iPad, and iPod touch - Apple Support