Make the five year old 8 or 9 and keep the seven year old the same. Of course, to some degree it depends on the story.
Back to book recs - my son enjoyed A Wrinkle In Time (series), Chronicles of Narnia (Series - be aware there i death at the end), The first 3 Harry P books (He can have book four in fifth grade), The Hobbit (read to him), The Wizard of Oz (Read to him in first grade, as much of the series as we coudl find), The Hardy Boys and Nancy Drew Mysteries, including Nancy Drew and the Clue Crew and the HB graphic novels, Encyclopedia Brown, Dragon Slayer's Academy, The Origami Yoda Books. I'm stopping before my fingers get too tired. He's big into fantasy and mystery but will read some contemporary especially if it's funny. His favorite book is Maniac Magee.