Feature Recommendation:
Considering how great Square's POS system is (I use it in several businesses/non-profits), and how versatile it is with regards to reporting and setup/use, I would recommend the following usage and features for Theater/Show sales...
Ticket Sales: Instead of having Variations of an "Item" ("Show") which shows "Stock" for each level of admission (General, Child, etc...), there should be one "Event" Item with Variations for each "Admission Level". Example: When selling a T-Shirt, you may have "T-Shirt" with Variations of "Small", "Medium", and "Large" with each size having Stock of 12/ea. But with an event, you have 400 Seats available at all 3 Admission Levels - "General", "Matinee", "Child". This will allow you to have set the total number of Seats for the event, regardless of which Admission Level - all keeping Inventory of the number of "Seats" (Tickets) sold. Would also be able to include a "Complimentary" Inventory to set a fixed-number of Seats towards Event at no charge.
Ticket Printing: A ticket printer with special thermal-glossy-high-weight rolls with perforated tear-offs would be an excellent way to print show tickets on-site with each transaction. (Ex: Citizen CL-S400DT Ticket & Label Printer) Each "Event" can have a separately-designed ticket design if desired, or just a basic location template with the event info updated. This function would also come in handy with the "Will Call" feature below...
In addition to Ticket Printing, there could also be an option (for a small fee) of a customizable "Venue Map" for "Seating Assignments". When a customer visits the Venue's website to pre-order tickets, the Venue Map could be used to pick seats for printing on tickets - and count towards the Seat "Inventory" sold. If this is made available, the Square Retail POS terminals would allow this to be show this map on the customer screen to allow them to select their seats before completing ticket checkout.
Will Call feature: When arriving at a venue, tickets can be printed on-site for pre-sale orders. Patrons could either pull up their receipt on their phone and the teller can scan that to pull up their order, or the teller could search for the name on the order, scan the credit card used on the sale, or the patron could present a print-out receipt with the barcode to scan. Instead of having a separate line for Will Call tickets, the box office could print their tickets to save time in the checkout process, and eliminate the need for staff to print/count/sort tickets and put them into envelopes and alphabetize them before a show.
At the entrance door, ushers could either have another Square station to scan customer tickets to "check off" the admission list to verify they were used, or a mobile phone with a Square app on it could be used to take a picture of the code on the ticket to mark the ticket as used.
One last feature for "Events/Shows" function would be an option in the menu for Concessions Stands: "Package" or "Combo" deals. This would allow you to pre-select a series of Items to be called-up automatically with a simple button push, but also count those items towards Inventory tracking. (Ex: "Combo 1" $12: Large Popcorn, Large Drink, 1 Candy Bar. This would add to the tally of sales for each item in the Reports. Same thing with Admissions "Packages".
Consideration should also be given for "Season Passes" or "Season Tickets" to allow patrons to use a Season Pass towards certain shows online or at the box office; using a pass towards a certain show would count towards "Seats" sold, and also show which Season Passes were used.
Square POS is already amazing for concessions items and general restaurant use, and have been adaptable for many other uses I have discovered over the years. These feature refinements for these type of items would make Square a complete multi-faceted system for a larger industry, bypassing many of these expensive systems that venues use and do not include many of these features.
