Here's an interesting article about the series and impact on reading in kids:
http://www.scholastic.com/aboutscholastic/news/press_07252006_CP.htm

It's hard to stop them once they start reading! We managed to hold off until age8 for book 1, with the hope of stopping and waiting awhile for the next ones. Well, he breezed through books 1-3 in a matter of weeks so our plan was blown away! We made him wait until this year for book 4 (age9) now he's been begging for 5. I'm worried about 5 since Sirius dies in that one-I think this will really affect him so I am holding off on that one as long as I can! Sensitivity comes into play with books 4-7. Good luck in the new venture!