I just skimmed through this and what I watch/read/listen to etc., has been on my mind lately. We could go on and on about why it's bad or isn't bad to read Harry Potter books. Without going into all that you should think to yourself all the forms of childhood entertainment that involve magic and/or fantasy. Do you think these are wrong? I'm not implying anything either way, I'm just mentioning that if you're going to make such statements that imply that any Christian who does____(Insert form of entertainment/media/anything that can be consumed in anyway) is either not really a Christian or isn't a "good" Christian, you should examine yourself and what you do first. I think you'll find that there's a lot that you need to examine before you make such brash judgments.
When I discuss with people my thoughts (that I've had very recently actually) about listening to certain types of music and being involved in things of the world I don't make a judgment or say "well if you listen to that song you're a HEATHEN, be gone!." The idea of these discussions is find out how we can be the most Christlike and glorify God in this fallen world recognizing our weaknesses. When we discuss with others it should be not to condemn, not to judge, but to HELP them. When we judge and/or condemn people, it tends to be out of weakness and insecurity and most of us, if not all, are guilty of it. It's one thing to point out sin that is explicit in the Bible, or even if we're pretty sure that one's behavior isn't Christilike. That said, correction out of love is a lot different than judgment. Who are you to tell someone they're not a good enough Christian?
With all the above said, I have read the Harry Potter books and I simply saw them as fantasy. Nonetheless, I can see the argument for not reading them.
Matthew 7:1