Agreed.
That's the one drawback to the series ... new readers to the series can pick them up back to back. A seven-year-old reading Sorceror's Stone is wonderful. I'd encourage them to read a few other things in between each book, however, so that would put off some of the worse stuff for a year or two. Even then, well, it still might be read along time.
Or it could be read one per year?? A seven year old would probably find Harry Potter a challenge... So it’s something that a parent and child could read together... Four or five pages before bed a night perhaps??