To shut down the iPhone, quickly press and release the Volume Up button, then the Volume Down button, then press and hold the Side button. Your phone will show a “slide to power off” bar at the top. Slide that, and it will turn off your phone.
How do I turn this phone off?
https://www.youtube.com/watch?v=exUygjUqqFw
Is there an app to turn off phone?
Google's Family Link app allows you to schedule a span of time for the phone to be off, such as bedtime. But you can also completely lock the device so kids can't even get into it. Family Link requires two downloads, one for your phone and one for your kid's.