“Kids shells” almost certainly refers to a basic pasta dish, “shells and cheese”. It’s just macaroni and cheese but with shell pasta instead. And the bill denoting “kids” likely means it’s a smaller portion than usual. Quite possibly the cheapest item on the menu to produce. The price for this item is insane.
I’m not sure what you’re asking with the 12 or 20.
Edit: ok guys I got the joke, thanks for the subtle hints