WARNING - THIS FEATURE IS CURRENTLY BROKEN ON iOS 12.x
Apple are aware of this issue and are yet to resolve it since the release of iOS 12.x, it is a known issue that causes the iPad to sleep when in Guided access mode after 15 minutes.
Guided Access limits your iOS device to a single app and lets you control which features are available. You can turn on Guided Access when you let a child use your device, or when accidental gestures might distract you.
Set up Guided Access
Go to Settings > General > Accessibility > Guided Access.
Turn on Guided Access.
Tap Passcode Settings, then tap Set Guided Access Passcode.
Enter a passcode, then reenter it. From here, you can also turn on Face ID or Touch ID as a way to end a Guided Access session.
Start a Guided Access session
Open the app that you want, then triple-click the Home button.
If you want parts of your screen to stop responding to touch, use one finger to circle those areas.
Control which features are available
To turn off features or set a time limit, triple-click the Home or side button, then tap Options. If you don't see Options, triple-click the Home or side button again and enter your passcode.
Turn on the options that you want, then tap Done