This would be done using the Guided Access feature of the iPad. It is not a feature of Square Register.

"Guided Access helps you to stay focused on a task while using your iPhone, iPad or iPod touch. Guided Access limits your device to a single app and lets you control which app features are available."

In the Settings app, select General > Accessibility > Guided Access.

Enable the Guided Access switch to turn on the feature.

Set the passcode one needs to enter to get out of the locked feature.  This should be different then your main iPad passcode.

The Time limit is how long to let it run.  When it runs out, the ipad locks and the above passcode is needed to continue.

Switch to the Square Register app and triple click the home button.  This will start the Guided Access feature.

At the bottom are your options of what to restrict.  The hardware options is to restrict the sleep/wake button, volume buttons, motion, and keyboard.  The touch option disabled the whole touch screen so obviously that should stay enabled.  Then the time limit option if you want.

Another feature is that you can draw on the screenshot of the app to block access to it.  Not necessary for Register but could be used elsewhere to block accessing a menu or other section.

To get out of Guided Access, triple click the home button and enter the passcode.  It brings you back to the setup screen so click End in the top left corner.

For more details, click the link above.