I thought the good thing about reading is that the age inappropriate stuff will just go over their head and they can still enjoy much of the book. Unlike movie or TV that can expose kids to things they should not see.

That said, I have not started my son on Harry Potter despite being a big fan myself. For that matter, I have not even showed him Finding Nemo. He found the Penguins in Madagascar scary. I cannot imagine he would not be scared by Happy Potter.