That doesn't, however, necessarily mean that Harry Potter is a completely evil thing, though. Neither a mountain nor a sunset are "explicitly Christian" in terms of "ideological content," so to speak, but does that make them "bad"? Of course not. For what it's worth, J.K. Rowling is a member of the (Presbyterian) Church of Scotland. Presumably, her Christianity, then, would have some kind of presuppositional influence even in her constructed fantasy-world.
I would say that ultimately it comes down to individual choice and discernment. One should guide one's children in this matter in much the same way as they would be guided with regard to the time to start dating, or whether to home-school or perhaps allow them into public school, despite the potential dangers to faith there. It's simply too individual of an issue to make an ironclad rule.
If a child is prone to being led off in a million directions, irregardless of their ostensibly Christian belief-system, then chances are that danger might lurk in reading these books or watching the films (as in much other "non-Christian" material). If, on the other hand, the child is strong and persistently able to withstand any competing ideas, contrary to Christianity, then likely no particular danger would be present.
Watching the films didn't harm my Christian faith in the slightest. On the other hand, at an earlier point in my life, when my faith was not yet strong or fully-formed (to put it mildly), the movies quite possibly could have helped lead me astray, since I did, in fact, get involved to a considerable degree in occultic pursuits. The supernatural held a strong fascination for me (thankfully channeled later on into Christian supernaturalism). C.S. Lewis himself was also seriously involved in the occult in the period just before his encounter with the music and romanticism of Richard Wagner and a mythological sort of contemplation which he described as "Northernness":
Now, for the first time, there burst upon me the idea that there might be real marvels all about us, that the visible world might only be a curtain to conceal huge realms uncharted by my very simple theology. And that started in me something with which, on and off, I have had plenty of trouble since -- the desire for the preternatural, simply as such, the passion for the Occult. Not everyone has this disease; those who have will know what I mean [I do, very well] . . . It is a spiritual lust; and like the lust of the body it has the fatal power of making everything else in the world seem uninteresting while it lasts.At times as I watched these movies, I must admit that for fleeting moments I felt precisely this "desire" that Lewis refers to. It's very difficult to describe without getting very heavy and mystical and philosophical, but it is a definite kind of coercion. I'm able to push it down because of strong Christian faith, but short of that, I can easily imagine (given my own background) someone with a similar bent being drawn into things which are harmful to their souls: true sorcery, witchcraft, Wicca, etc.: things which are definitely wrong and condemned in the Bible.
(Surprised by Joy, New York: Harcourt Brace Jovanovich, 1955, 60)
And that gets back to our main issue. Witchcraft or sorcery (everyone agrees) is portrayed in these books and movies. The side of "good" is chosen and ultimately triumphant, so there is little question of an advocacy of evil things. But on the other hand, overt hints of Christianity or God are missing (as far as I can see; perhaps the books are different). If parents allow their children to take part in this craze, then it must be with caution and discussion about the relationship of these stories to Christian doctrine and the ontology of the real, spiritual world that we live in. Fantasy is just that, but we know that the devil can use all things to lead people astray: especially if they are not vigilant.
Ultimately, then, I am not "against" Harry Potter, yet (similar to even many proponents) I would strongly urge a sharp watchful eye for potential spiritual danger, due to the nature of the subject matter, in proportion to the extent that one is prone to following non-Christian modes of thought and behavior, contra proper Christian boundaries and mature spiritual discernment. My view is very close to that expressed by the British Evangelical Alliance.