Both the Old and New Testaments warn against it with utmost urgency. Occult activity is a misreading of the nature of the war between good and evil on this planet, and the consequences of this in real life can be quite dire. Fantasy literature can be a splendid way to introduce them to the great drama of existence, but we are terribly naive if we fail to make a clear distinction between true fantasy and false fantasy - between healthy imagination and poisoned imagination. Simply saying that the corruption of our symbol world, and in the worst cases the inversion of our symbol world, is not poison doesn't change the nature of the poisoned fruit. That's denial, not moderate reasonableness.
Parents often underestimate the power of imagination in shaping a child's sense of truth.
Parents forget that they themselves grew up in another time and culture. That is no longer so. Parents also forget that they can sort through good and bad material with more immunity than a child, because they are already formed. A child is still in a state of formation, and for that reason he experiences culture in a very different way than adults do. He is busy learning about the world, and usually he is learning indiscriminately.