Spinning Yarns That Deceive
Harry Potter books are not as dangerous as ones that directly undermine Christianity.
Think about it. Pullman claims he's trying to undermine Christianity. But the Christian faith doesn't appear in his novels. The god he depicts is feeble, not omnipotent. The religion he describes is petty and malicious, not the faith that inspired almost every significant humane and charitable endeavor in Western culture.
Pullman is using Christianity as a straw man to make his case. But no matter how flawed and incoherent these books may be, they are still dangerous. Just as writers like C.S. Lewis used stories to "smuggle" Christian theology into readers' minds, secularists like Pullman slip misinformed prejudices into the minds of unsuspecting--and uncritical--readers.
Obviously I don't recommend that your kids read these books. But their growing popularity makes it likely they'll run into them. So you need to take the occasion of these books' popularity to sit down and explain these issues to your kids. Preparing ourselves and our kids to respond to books like these forces us to hone our own worldview.
What's more, Pullman's books will likely be on the same shelves with tales of Narnia and Middle Earth. You've got to be sure that your kids and grandkids can distinguish the counterfeit from the real thing.