How to Lock Your iPhone While Your Kid Uses an App

Here’s how you do it

1. Go to Setting > General > Accessibility > Guided Access.

2. Turn “Guided Access” and “Accessibility Shortcut” on.

3. Open your kid’s app and press the home button three times in a row. (For iPhone X, triple-click the side button instead.)

4. The first time you do this, it will prompt you to enter a passcode to start the “guided access.”

5. To exit guided access, hit the home button three times again and enter your passcode.

Source: How to Lock Your iPhone While Your Kid Uses an App