I just learned this. If you go to Settings - Accessibility - Guided Access, you can create a pass code and then, after opening the app you want to be the only one available, press the home button three times. After that, the home button will no longer work, and they have to stay within that app.

It also lets you circle parts of the screen you don’t want them to be able to access. So you could start a movie in Netflix and disable the back button at the top left, preventing them from selecting a different movie.

To exit Guided Access, you press thr home button three times again and then select the End option, entering the pass code when prompted.