Making judgment calls about what level of involvement we should have in "non-Christian activities" is never easy. The Puritans, and many others, were extreme separatists when it came to "worldly" activities. The theater, playing cards, and dancing were all considered activities from which a true Christian would abstain. (They even outlawed Christmas because of its Roman Catholic trappings!). A few conscientous believers today would take the same approach to television and movies in general and books like Harry Potter in particular. For believers who feel thus persuaded, I can only say, "But whoever has doubts is condemned if he eats, because the eating is not from faith. For whatever does not proceed from faith is sin" (Rom. 14:23, ESV).
Honestly, as I look at my own walk with the Lord over the years, I've been all over the map on questions of Christian liberty. At one time Holly and I sold our TV and lived "TV-free" for close to two years. (We did not totally quit going to movies or renting videos to watch with friends, but it was a severe cut-back from our previous habits). We now own a TV, VCR, and DVD player and while we don't watch a lot of TV, we do sometimes, and we frequently enjoy watching movies together. On "gray" issues like this, each believer must decide for himself or herself where the limits of liberty are. Of course, some things are clearly sinful. I cannot say that drinking beer is sinful, but drunkennes certainly is. I cannot say that watching television is sinful, but watching pornographic videos certainly is. And I think that believers who walk close with Jesus and are saturated with his Word know when the line is crossed.
As a parent, I've been helped by thinking of my own parent's way of handling things when I was a child. When I was six years old, Star Wars was all the rage. My cousins were playing with Star Wars action figures and The Empire Strikes Back was released in theaters. I was just the age to be totally enthralled with it. My parents were cautious, especially at first. I'm sure they had reservations about their six-year old playing with grotesque alien action figures and watching films with a worldview that had more in common with Zen-Buddhism than Christianity. But this is where they landed: they allowed me (and my brothers) to play with the toys, and even see the movies. But they also educated us. They carefully explained that "the force" was not real and that they didn't like the way the characters in Star Wars said, "May the force be with you" (which sounded a whole lot like "God be with you"). Well, I really like Star Wars to this day and am glad my parents were not too strict in their rules. I enjoyed literally hours of play with my Luke Skywalker and Darth Vader action figures! But I am especially glad that they taught me to evaluate things in light of a Christian worldview. Of course, I didn't really know this is what they were doing at the time. But they were modelling. They did the same thing several years later when as an eleven year old I discovered Edgar Rice Burrough's Tarzan of the Apes novels (which were good and relatively clean, but not without their pagan elements and occasional sensuality). They allowed freedom, but also taught me how to think, how to evaluate, how to ask questions, how to measure the rightness and wrongness of ideas by Scripture. And I really do believe that this was more valuable for my spiritual formation and maturity than a simple rule forbidding Star Wars movies or Tarzan novels would have been.