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.