You can do one of two things or both.  On ours I use Restrictions.   Settings app / General / Restrictions.   Just use a passcode you will not forget.  In restrictions, you can shut and or lock a bunch of stuff.  This limits what they can do with the iPad.

You can also use Guided Access.   Settings app / general / Accessibility / Guided Access.   Turn it on set the passcode to something you will not forget.  Ignore Time Limits settings.  Turn on Accessibility Shortcut.  Now launch Square,  Then tap the home button 3 times, this brings up guided access.   Under hardware turn on the buttons, you want them to have access to.  I would turn them all on.    Make sure touch is turned on.  Now hit start.  Now you will see that pushing the home button does nothing and give a message at the top.  Do shut guided access off press the home button 3 times and enter your passcode.  Click end to end Guided access.

Hope that helps