The easiest way I know to see if a book is too hard for kids. Works best with a 100 word passage. You can teach children to do this themselves and monitor their reading. Five Finger Rule Poster (Free printable)
This year I decided to combine the retelling roller coaster with the 5 finger retelling rules, because they go hand in hand. Now the kiddos know to use their fingers and when they get to their ring finger they know to do the roller coaster and remember details from the beginning, middle and end of the story.