Haha that reminded me that there is a tiny maritime Restaurant in a town near where I live, where they have giant meal sizes. If you want a normal sized portion you'll have to order a kids meal and the waitress brings you a coloring book and crayons. If you order a normal portion and don't finish it, they'll ask you nicely force you to donate to the maritime rescue organization.
I could totally see how that mistake would be made. I’d be so fucking horrified as the server but if the place was busy and I just caught a glance, height is the most obvious indicator. It might not have even been by the person seating them or interacting, maybe just a server that thought they were being helpful. Big Oof.