Five Finger Rule. This is also another great idea for students when it comes to picking out books they want to read. However, you should encourage reading different material instead of books that they can easily understand. They may need help with the books but they are able to look at different books. This can be used for when they are picking books out individually.
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)