As children begin to read outside of their reading levels, they will stumble onto words and references they may not yet understand. Like people in Spanish classes who hear only Spanish and are allowed to speak only in Spanish, they must force themselves to learn what certain words mean to understand what the instructor says.
Reading at a higher level may improve a child's vocabulary also through the associations he makes between words. For instance, if he comes across a word that he doesn't understand, the context in which it's presented may enable him to grasp some sense of what the word means.
Some opponents of forcing children to read at a higher reading level find that making a child read harder-to-understand material will create resentment and boredom. If the child does not understand the words or concepts presented in the book, he will not enjoy the act of reading and may never want to read another book. As Dashka Slater of Babble.com suggests, a child reading ahead of his level may toss books aside, as they probably will be over his head.
Slater's example consists of reading to her first-grade son the "Harry Potter" series. During the first two more light-hearted books, the boy loved listening to the book read aloud. However, when the third, darker book, "The Prisoner of Azkaban," was read, he quickly became bored, because the book's themes and innuendos were beyond his comprehension.
When children take on reading levels that are higher than recommended, they will delve into material that could end up taking away a piece of their childhood. Realistic books about growing up, like "Blubber," for instance, when read by a fourth-grader, will force her to see a world you may not want her to see at such a young age. Though the child may understand all of the words on the page, she might not be ready to tackle such themes as death and meanness.
Before your child begins a book at an advanced reading level, you may want to make sure that the topics explored in the novel will not shell-shock her into realizing darker things about the world than she ever knew before.