There are great dissimilarities between Rowling and Tolkien/Lewis and I would not take this statement at face value until you have done some additional research.
When we say that Harry Potter could be dangerous for children, what do we mean? I propose the following list of dangers:
- That children might become convinced that occult powers are real
- That children might become tempted to try the occult in the form of spells, potions, etc.
- That children might become convinced of the innocence of the occult
- That children might become ambivalent towards morality
An overview of the Harry Potter world:
In the world of Harry Potter, seemingly ordinary people wield magic, often in hiding from “muggles” (ordinary people). The story is set in the real world (London to be precise). Otherwise normal children are discovered to have powers, which changes their lives.
An overview of Middle Earth:
Middle Earth is not like the real world. It isn’t a magical world that exists hidden from view, which exists side by side with the real world. Who wields magic in Middle Earth? Wizards, not normal people at all - legendary figures like Gandalf and Saruman. There is no way for children to aspire to be like them, because they are never portrayed as normal children. From the get-go, the story is obviously myth.
As an afterthought, but perhaps still important: Rowling’s hero character, Harry, tells lies throughout the entire story. He’s sometimes punished, but more or less encouraged even by the supposedly wise Dumbledore. The danger here is clear: children, after reading this, may believe that lying is permissible when done for the right reasons. Parents should remind them that lying is always sinful.
Again, these are just some points to consider when discerning whether a particular book presents a danger to children. You might arrive at a different conclusion, or you might be inclined to agree.