Satan will interfere to children's literature, movies and cartoons because their souls are vulnerable. The soul of a child is innocent while adults are corrupted. Innocense in a child is fragile because it has not matured, satan wants to break the innocense to corrupt it. Since satan is dirty, to him seeing something clean irritates him.

He needs to act so that he won't spend eternal hell with just a few numbers of his followers. He wants to try to get a lot of people as much as he can to hell. That is why he would do that. And he would do that by making you think that magic, fantasy, rock music, philosophy is ok. He does this through his creativity in making you think and see that the good in Harry Potter is through magic. He wants you on his side because you think what you see or believe is good. To him the more the merrier.