Thanks to a couple of nifty features, you can now set your iPad to prevent access to other areas such as games, the internet and iTunes.
If you wish to go one step further and lock the iPad, so your waiting staff may only utilise iZettle Pro, then you can enable a feature called 'Guided Access'.
'Guided Access' can disable all of the hardware buttons, including the home button, preventing users from exiting the app unless they triple click the home button and enter a pin code.
Restrictions can be found by going to 'Settings > General > Restrictions' on the iPad itself.
'Guided Access' can be also be found by going to 'Settings > General > Accessibility > Guided Access' on the iPad.