There have been many rumored "rise in interest in the occult" from time to time, supposedly triggered by everything from the old "Bewitched" and "I Dream of Jeannie" and "Dark Shadows" TV shows to heavy metal rock bands. It's mostly bunk, fueled by a certain amount of panic whipped up by those with an axe to grind. There are a few kids who are drawn to the Wiccan culture and stick with it, but they are few and their numbers don't seem to change very much over time (and frankly, better that than the Goths, who are often Satanists -- and the Wiccans are more afraid of Satanists than your average Christian, simply because they know more about them and what they do.)
As for how I know, I've known a Third Degree priestess in a Wiccan "grove" (not coven) for something like 25 years. (I think she is in grievous error, but she was an atheist before she was a Wiccan, so at least she's moving in the right direction.) Over the time she has been associated with her group (which is the largest in our area), the number of people interested in the Wiccan religion has remained fairly flat (and small). The group doesn't proselytize, and they don't allow minors to join unless their parents are already members. So I just don't see this as that imminent a threat.
First, I hope we can all finally agree that this book series encourages chldren to further "investigation" into "witchcraft," whatever that term means to TV-educated ten-year-olds.
Secondly, I generally concede your point. But that doesn't settle the issue. There are many children (especially from broken homes) who are desperate for control over their lives. For the spiritually ignorant (the majority of children these days?) sorcery can represent a means to achieving control and power. While some children might drift away from the occult when activities don't go as planned, other children might delve in more deeply.
I've heard several interviews with the exorcist for the archdiocese of NY. He states that playing with ouija boards, holding seances, and the like has opened the door to the occult for many children, paving the way for demonic obsession or oppression. It's not unreasonable to assume that Harry Potter can do the same thing, or at least serve to open the door to ouija boards, holding seances, etc.