Traditionally, most libraries have a standardised categorising system. Fiction works are shelved by author's name, and non-fiction works are categorised according to the Dewey Decimal System, a numerical system where books are given a number based on their subject matter. For children's books, however, many librarians focus on creating a classification system that is easier to use for the younger set.
Separate books by age level. Books that have few or no words are appropriate for babies and toddlers. Picture books with fun stories are appropriate for preschool and young elementary-age children, and simple chapter books are good for advanced elementary-age readers and middle schoolchildren.
Sort toddler picture books by author last name, to make them easier for parents to find. It's also often convenient to place these books in bins, rather than shelves, so little hands can reach them and see the covers when choosing a book.
Separate books into fiction and non-fiction works for preschool and elementary-age children. For fiction books, sort and shelve by author last name. Non-fiction books can be sorted according to the Dewey Decimal system, but it's often easier for children if books are sorted by subject. For example, books could be sorted as books about animals, books about weather, books about families, and other categories based on the types of books available. With the younger school-age crowd, bins are a good alternative to shelves as they provide easy access to the covers of books.
Separate books for older elementary-age students and teens into fiction and non-fiction works. Non-fiction should be categorised using the Dewey Decimal system, as it's an important system to learn. Fiction books should be categorised by author last name.