Does the charity round-up show as a discount on my numbers?

No. A round-up is money the customer adds, not money you take off. It is itemised on its own line on the receipt, kept out of VAT because a donation is not a sale, and your revenue is reported with the pledges already removed. The Games & extras page under Marketing shows Pledged by customers, which is what you owe the cause you named. The money reaches you with the rest of the order, in cash or on the card machine, and it is called out on the ticket so whoever counts the cash knows part of it is not takings.