I want to present a respectful supplement to his list of 13 books he recommends children read over the summer as an anecdote to sliding comprehension and retention of what they get in the classroom. It's not a bad list, per se. But 9 of the 13 books he recommends have male main characters (all due respect to Hermione Granger, and I am glad someone is resurrecting Freddy the Pig), one of the female-coded characters is a linguistically-gifted spiders, and Kristof picks only one of the classic books about girls, and in my opinion, one of the weaker alternatives: Anne of Green Gables. So first the obvious suggestions:
1. Little Women: Leaving this book off a recommendations list for children is insane. It's grounded in Civil War history, has great examples of literary forms embedded within it, has four beautifully developed heroines who mature in very different directions, and though the morals about the value of work are a touch heavy-handed, they're still extremely valuable morals.
2. The Little House on the Prairie series: Ditto for this series, also rooted in the sweep of pre-Civil War history, but from further out West. The books are tough (the scene in The Long Winter where Pa shames Almanzo and his brother into giving him wheat to feed his starving family is remarkable), tender (Laura's sister Mary's blindness, and her triumph over it separate her from her family in certain key ways), modern (Laura refuses to say she will obey Almanzo in their marriage ceremony) and full of terrific male and female characters.
And some less-obvious ones:
3. Caddie Woodlawn: Caddie's very much in the moral tomboy tradition, much like Jo March. But the reader gets to see her warn her Native American friends of an impending attack by her family's white friends and neighbors, and her family later votes that her father should turn down a noble title and estate he's inherited from an English relative to stay in America. Even though she's a child, Caddie's aware of the contradictions that surround her, and she makes active choices about which side she's on. (Kristof mentions this one in a follow-up, as he does the Little House books, though he misattributes Farmer Boy to Almanzo Wilder--Laura Ingalls wrote it, not her husband.)
4. Patricia C. Wrede's Enchanted Forest Chronicles: Princess Cimorene is simultaneously one of the great parodies of the fairy-tale princesses and a snappy, fully-realized character in her own right. As a failed princess who runs away to go work for a dragon rather than marry a prince who annoys her, Cimorene (and Morwen, the deeply practical witch who becomes one of her best friends) has the guts of a crusading prince (or knight, she threatens to fight both in the books), the sense of humor of a court jester, and mad skills at making Cherries Jubilee and chocolate pudding, both favorites of her employer and eventual (female) King of the Dragons, Kazul.
5. The Trixie Belden books: If Nancy Drew bores Kristof, he should check out Trixie's adventures. Set in upstate New York, Trixie and her best friend Honey solve mysteries that range from hidden fortunes to horse doping at the Saratoga race tracks. The danger is often real, and the contrast between the agrarian middle class, represented by the Belden family, and the rich, represented by Honey, isn't heavy-handed.
6. Philip Pullman's Sally Lockhart mysteries (particularly The Ruby in the Smoke, The Shadow in the North, and The Tiger in the Well): Before the His Dark Materials trilogy, Pullman created an incomparable heroine in Sally Lockhart, who opens her own investment firm for female clients, kills an industrialist who has invented a weapon that will help governments massacre their own people during uprisings, and marries a Jewish socialist.