I think frum children greatly relate to the idea of being part of a discrete minority whose activities are largely hidden from the general public and totally misunderstood by the general public, and who are also subject to sometimes severe discrimination and persecution. Much of the lives of frum Yidden in golus are very much parallel to the “wizarding” subculture in the Harry Potter novels (i.e. we are the wizards, the goyim are the muggles).
There are some Christian themes that the author included, but it should be noted that none of these involve Avodah Zarah, and are based on aspects of Christian culture that were derived for Judaism. If a Yid had written the books, they could have used the same concepts and we would attribute them to Yiddishkeit origins and history. The “witchcraft” in Harry Potter doesn’t involve looking to or believing in an Avodah Zarah, and should be considered alternative science (consider the famous line to the effect that any technology too advanced for the observer to understands appears to be witchcraft). Advanced beings with superpowers are only a problem if the superpowers are derived from an Avodah Zarah (that there is nothing “treff” about either Star Trek’s “teleportation”, or Harry Potter’s “apparation”).