If it wasn't enough that Harry Potter was teaching kids to seek out black magic, now we have Amy Welborn ragging on The Wizard of Oz. Here, I think we definitely have run into a problem.
Fictional books and movies which seek to teach about religious belief are uncommon; those that intend to do so for children are quite rare. Much more commonly, it is the virtues that are the subject. So it is, clearly, with this movie. And in the scene with the wizard (which, by the way, is patently supposed to be preposterous) the message about the virtues is simple: you are what you do. Such a simple message, in fact, that Jesus teaches it in the parable of the sheep and the goats. Oh, and about self-reliance.
Ah, there's the rub. The difficulty is in the translation: "Oz the Great and Powerful" is a symbol for God; therefore the movie teaches that God is a humbug. Well, maybe. And maybe not. The symbol is more complex than that, for one thing. After all, their "prayers" to Oz-the-Powerful are answered-- just not in the way that they expected. How God-like! And further along in the movie-- well, once the balloon appears, God-symbolism is completely out the window.
The bigger issue seems to be this: in a secular movie, it's easy enough to teach against virtue, by accident or on purpose. It's easy to teach virtue on purpose. But it's exceedingly hard to teach specifically Christian principles, and next to impossible to direct people to interpret what they are seeing along specifically Christian lines.
With respect to Harry Potter, you have to be pretty thick not to notice that explicitly Christian symbols appear all over the place.