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/lock button. This is the small button on the short edge of your iPad, so you will need to remove your iPad from its mount to do this. On older model iPads, this is the large round button under the iPad screen. 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/lock button again, enter your Guided Access passcode, and select End.