Those kids are at MOST in 3rd grade.

Here are the approximate word counts for each book in the Harry Potter series by J.K. Rowling:

Harry Potter and the Sorcerer's Stone: ~77,000 words
Harry Potter and the Chamber of Secrets: ~85,000 words
Harry Potter and the Prisoner of Azkaban: ~107,000 words
Harry Potter and the Goblet of Fire: ~190,000 words
Harry Potter and the Order of the Phoenix: ~257,000 words
Harry Potter and the Half-Blood Prince: ~168,000 words
Harry Potter and the Deathly Hallows: ~198,000 words

In total, the series contains around 1,082,000 words across all seven books.

They would have had to read the entire Harry Potter series. They would have read “War and Peace” twice. They would have read about 80 Capt. Underpants books. They would have had to read about 50 Goosebumps books. A book for a 3rd grader typically consists of about 20,000 words, so in 10.5 months (school year) they would have to read almost 5 chapter books a month. Impressive at that age.