By the way, I totally understand if you don't want to answer this, especially given that this subreddit is generally filled with privacy enthusiasts (you know, the whole point of Monero) but...
Are you willing to say where your restaurant is? I'd go if it's near me.
The guy who said you need to trust the employees is wrong.
Put a view-only copy of a wallet on the register computer (or mobile device if you don't have a PC as a register) and use the GUI wallet's built-in Merchant menu. It should be intuitive and quick for both the cashier and the buyer.
If you want more help with this, just respond to this post and I can explain more thoroughly.