It's implied by Quirrel that nobody even knows what an Oni is, nevermind their nutritional requirements, or that Suika even is one. Which makes sense, the British Wizarding community barely knows what electricity and cars are and their seat of government is under a modern London street accessible by phone booth, nevermind some island-locked local 'troll cousin' from halfway around the world where people don't even speak English. As far as anyone local is concerned, Suika just has a weird accidental transfiguration of horns on her head, and is otherwise just a normal human child (if a really fucked up alcoholic one that they should really be putting effort in to getting mental help for, but you know. Wizards.)