The feature is called screen pinning, and it’s been hiding in Android’s settings menu since Lollipop. (Look under “Fingerprints & security.”) Turn it on, and then you can “pin” a single app to the screen whenever you like.
While the screen is pinned, the kid can’t get out of the app with the usual back, home, or overview buttons. (That’s what the square button is called.) This feature is also handy if you want to keep a certain screen open but worry that you might bump a button by accident.
See http://lifehacker.com/your-android-phone-has-a-built-in-childproofing-feature-1794809851 
Your Android Phone Has a Built-In Childproofing Feature
Ever needed to hand your phone to a kid? Even if you don’t want to, sometimes you’re trying to keep them quiet at a restaurant, or calm them down at the doctor’s. But handing over an unlocked phone is just asking for the kid to delete all your home screen shortcuts (or, worse, work emails). Fortunately there’s a way to fix this.