By using guided access, a feature that comes with iOS on the iPad, you will be able to restrict usage to one app.
This feature is accessible by opening settings on your iPad, going into General, choosing Accessibility and then selecting Guided Access.
Enable Guided Access and set a Passcode. This Passcode will be needed to end the Guided Access session so keep it some where safe.
Once you have opened the Snappic App, triple tap the Home Button on the iPad and press Start, this will turn on Guided Access.
Now when your guest tries to leave the app they will need to enter the passcode in order to do this.
Below is an article that will assist with Single app/kiosk mode