Press and release the Sleep/Wake button, the screen should go dark immediately.
In sleep mode the kiosk will use minimal power. However, it is still on and using some battery power even though the screen is black.
This is also considered locking the iPad.
Wake or unlock
Press and release either the Sleep/Wake or Home button. A “Slide to unlock” bar will appear on screen.
Slide the bar to the right and the screen will be unlocked. Newer iOS versions will ask you to enter in a password.