To stop your guests getting out of the app and getting to the main table you need to kiosk app. I recommend Fully Single App Kiosk from the google play store.
Can be found here.
This is not a free app to use but it is the only one that works i could find. There is a one off fee of 7.90 EUR per device plus VAT. This is need as it will remove the water mark from the app.
Once you have set up Kiosk app make sure you have the startup screen to Donation Options or Jukebox in the setting screen on the SumUp Donations app so when the Kiosk app reloads it go back to the set screen and not to the setting page.