Guided access via the accessibility shortcut is a good way to force the screen to stay on. Go onto the accessibility settings, enable Guided Access, and tell it

Guided access via the accessibility shortcut is a good way to force the screen to stay on. Go onto the accessibility settings, enable Guided Access, and tell it to activate when you triple click the power button. Then when you’re in an app and you don’t want it to turn off, triple click the power button, select options, disable the power button, and start. It’s also great for locking other aspects of the phone if you’re going to hand it to someone else for example.