With Shopify’s Shopcode app you can generate QR Codes for your store that act like a physical buy button, or you could use them online to direct customers to a product or checkout page. The QR codes you generate with the Shopcodes app doesn't change anything in your Online Store, affect your customers, or change your orders.
https://www.shopify.com/buy-button (combined with a lite plan can let you segment parts of a business.)
https://shopify.github.io/buy-button-js/ to develop custom interfaces.
Out of curiosity would you invest in hardware to basically have a locked down POS with card reader at each table?