Some kids read better than others. And some kids speed through everything directed to children and need further material. What to do?
Enthusiastic readers will eventually outrace the usual children's fare. This can be problematic for a couple of reasons I will discuss later. The first order of business is determining whether your child is at this point.
In the library, the arbitrary, customary and theoretical and division between a Child and a Young Adult is approximately age 12. This is arbitrary because, as most parents know, there is no wave of a magic wand and *poof!* you're a young adult moment. And trust me when I say that the line between children's books and young adult books is more a chasm than a line.
In the children's section, Juvenile Fiction is the classification for chapter books, and these can range from very easy to fairly challenging. As a rule of thumb, kindergarten through third graders will be reading from a section of beginning chapter books called Easy Readers, so I am speaking of books aimed at the older kids, aged about 9 to 12. Reading skills vary widely, so that 7 year olds may be reading on a fourth grade level, and 7th graders may be struggling along in much younger books. Parents should have a sense of where their child falls in these parameters. In this discussion, we will deal with the more advanced readers.
Within a children's department at the public or school libraries, you should feel reasonably safe having children read any book on the shelves, with a few caveats. If you know that your child is more sensitive to sad, scary or otherwise intense situations, you might want to screen books using reputable sources. These can include the librarian, older siblings who have read the books, and reviews by professional reviewers. One clearinghouse website is BookSpot (http://www.bookspot.com/booksforchildren.htm) where you can see several links for evaluating children's books. Another valuable resource is a database called Novelist. It is a paid subscription site, but your public library is very likely to have a subscription which you can use from home (call your library for details). It is important to remember that, while children's books are monitored for rough language, gratuitous violence and the like, there are still apt to be some hard subjects for younger kids, such as hardships of war, holocaust, death, loss, divorce, etc. More important than just knowing what vocabulary your child understands and whether they can comprehend a story, is determining what kinds of information you are comfortable having them exposed to at this young age.
Generally, the problem for advanced readers is that they do comprehend very well, and have large enough vocabularies to require more of a challenge. They have read increasingly more involved literature until they reach the point at which they perceive some books as too formulaic, too predictable or not complex enough to interest them. The temptation may be to cross them over to young adult books, but this move should be thought out carefully. Young adult books often include themes that are usually not present in books for younger children. These can include drug abuse, sexual situations, graphic violence and language, and more. Information which can be cautionary for older kids may be downright frightening for younger ones.
Some series books appear in both sections of the library. A good example is the Harry Potter series. You may have noticed, or heard from others, or observed in the movies, that the storyline became a bit more intense as time passed (and with each ensuing book in the series). Hence, the earlier Harry Potters appear in the children's department, but the later ones are to be found in the young adult department. These designations are not accidental. Much review and debate goes into the decision of where to place books in the system. Long and short - be cautious in sending your 8 year old prodigy reader over to YA for books. Timmy may understand very well what he is reading. Perhaps better than you might wish.
So what can they read? Like it or not, this is where you as the parent may have to take a more active part. You can start by recommending books you have read which you find appropriate. If your child's tastes are different than yours (and this is almost certainly the case), you might research in the sources listed above for appropriate material. If your child is older and you want to carefully introduce books of a more controversial nature, you may have to read them also. Your child's mental development can be no more important to you than their social, spiritual or emotional development. If you can talk through issues, you will know when he or she is reading something they find troubling.
Children on the older end of the scale, say 12 or 13 years old, may be able to handle many of the classics, such as Dickens, Austen, Twain, and etc. You may need to be available to explain a few issues having to do with Victorian society (Dickens) or slavery (Twain), but these can be meaningful conversations.
If in doubt, ask the librarian. Especially in larger libraries, children's departments are staffed with folks who, by training and/or experience, are good at being up-to-date on what kids are reading.