Is it OK to use secular media to teach Christian principles?
It certainly is possible in discipleship to take secular things and use them as illustrations of common grace or evil and to teach lessons from them. So, in principle, I can imagine that one might do that.
The danger is this: Is it the most helpful thing to do? Is it the best way to nurture a radical holiness, a radical love for Christ, a radical allegiance to him, and a radical commitment to serve, sacrifice and love? Or is it just playing with the culture in such a way that we merely tantalize young people and justify some of their already existing worldly inclinations?
This calls for tremendous wisdom in the life of our youth leaders. I would be disinclined to think that, in broad and general ways, Harry Potter books and movies would be the best discipleship material to use for drawing out lessons. If they were an unusually gifted group of young people who were bent on the artistic development of their skills, and you wanted to use a Harry Potter film as a piece of analysis by which to understand the world or evil, then sure. That sort of thing is going to be done in higher education and all the way along.
You don’t have to avoid engaging with culture if your commitment to analysis is radically biblical and you mean to really distance yourself from anything unbiblical that you may find there.