Fun fantasies and strategic entertainment have a special way of altering values, compromising beliefs and changing behavior in adults as well as in children. This learning process has been named "edutainment" a favored way to educate and entertain at the same time, for example Sesame Street. Its power to influence beliefs and behavior is illustrated by today's marketing firms; which spend billions on entertaining ads that touch emotions, ignore the facts, and yet motivate people to buy the product.
This Harry Potter 'ideology' is a good example. It comes complete with trading cards, computer and other wizardly games, clothes, action figures and cuddly dolls as well as audio cassettes that could keep the child's minds focused on the occult all day and into night. What's next, a Ouiji Board?
Teach your children Allah's Truths and train them to see reality through the teachings of Allah and His Prophet (peace be upon him) and not through the world's perspective. To succeed, we must be ever-conscious of their surroundings. Is it possible to shield them from contrary values, I'm sure it's not, however it is possible to offer them clear understandings of these things in light of the Qur'an and Sunnah. Once they have learned to love what Allah loves and hate what Allah hates, and to see life from His Words, they will demonstrate their wisdom by choosing to say "no" to Harry Potter and the like.