Doesn't matter. Spoodles' price for fettucine Alfredo is $16.79. The plain pasta that the OP's daughter had was the this entree without the sauce. Nobody would expect to order a Caesar salad with no dressing and be charged less than the menu price; nobody woud order any entree "dry", aka without whatever sauce, gravy, reduction, whatever is listed in the item's description and expect to pay anything less than the menu price; why is it so surprising that when the young woman ordered a menu-listed entree without any sauce, the restaurant still charges $16.79 for it?
Again, Spoodles HAS a pasta entree on its menu. It doesn't matter what the restaurant's cost is for a portion of pasta, they charge $16.79 for it - with or without the Alfredo sauce.
See above. Spoodles doesn't sell an adult entree of plain pasta. Instead, they legitimately charge the menu price for their pasta entree, with or without sauce. As the Guest in question is (or was at the time of the meal) eleven, aka a Disney adult, she was - I would expect - served an adult-portioned entree, so the price of THAT is what was on the check. Now, armed with this knowledge, next time the parents would probably be better off paying out of pocket for her and having the daughter order from the kids' menu.
So you wouldn't charge the price listed on the menu? Remember, no matter what you consider a child, Disney considers an eleven-year-old an adult for all pricing purposes escept rooms.
Well, since you asked, yes, I do have some training - but since it's not current, I'm not going to argue cost vs. profit. I'm simply explaining why a restaurant would charge the same price for a given menu item, with or without sauce.
Using two popular fast-food restaurants as examples:
When I was a kid and we got McDonald's (yes, it was around THAT long ago

) two of my siblings would eat only absolutely plain filet o' fish sandwiches. No cheese, no tartar sauce... but no discount on the price, either.
And, afaik, Burger King still offers "have it your way", right? They don't (or didn't, it's been ages since I've been in one) charge any less for a Whopper plain than they do for a standard one.