It depends substantially on what we mean by "dark arts." Generally, I think of dark arts as persuasion techniques that are effective at changing someone's mind without relying on that person's rationality. The important lesson is that deliberately circumventing the rational process of another is wrong.

But children (especially young children) just aren't rational by adult standards. So, is it a kind of dark art to take advantage of some irrational thinking process for the purpose of improving little Johnny's future rationality? It's deliberately avoiding the (mostly non-existent) rational process of the child. But it doesn't seem wrong.

That said, the example about changing aesthetic preferences is hard to justify on "improving future rationality" grounds. Which doesn't necessarily make it wrong, but it certainly is a closer question.