Once you have enabled Guided Access in the iPad Settings, you need to start the NoahFace App in Guided Access mode.
To do this, start the App as normal, and then, once it is running, triple-click the iPad home button. When you do this, you will see the text Guided Access Started appear in the middle of the screen for a few seconds.
If you later want to exit Guided Access mode, you just triple-click the home button again, enter your Guided Access passcode, and select End.
Note: If you are running an iPad Pro, then Guided Access is started by triple clicking the power button.