Press the Top button
Press the Home button if your iPad has one
Use AssistiveTouch and tap Lock Screen
Set Auto-Lock in Settings
Go to Settings
Tap Display & Brightness
Tap Auto-Lock
Choose a time limit
Use Guided Access if you want to keep the screen locked in one app
Go to Settings
Tap Accessibility
Tap Guided Access
Turn on Guided Access
Triple-click the Top button or Home button to start it