You can adjust your iPad to sleep and/or dim while running The Receptionist with the additional options under the Device Settings menu in the app.
To get there touch the top middle of the home screen in the app for about 6 seconds (the active area you need to touch and hold is highlighted below in red):
Then click Device Settings and you'll be prompted for your password:
Once you enter your password and confirm, you'll see a new screen where you can change the brightness and sleep/wake functions:
Please note this feature is available in version 4.6.0 of the app or newer.