At its simplest, a bookshelf is an open-sided box with shelves inside. Even in this most basic form, it makes more sense to make your own. Since most kids’ books are thin, you don’t need as much linear space to hold books; it is much more important to make the space between the shelves taller or shorter than normal.
Unless your child has a large number of books, you may want to build a bookcase that looks more like a magazine rack. Large thin books can then be displayed face out, where your child can quickly find that one book he loves most.
Turn that boxy bookcase into a dollhouse. Paint it a different color and add a pitched roof--just a couple of triangles glued to the top of the bookcase, with two roof panels on top. Add windows and doors to complete the look.
You also can make a tall narrow case to serve as a skyscraper in a “storage town” where the child’s other belongings can be kept.
Make the bookcase part of a train. Build a play engine for your child to sit in, perhaps with a small reading table, and hook the bookcase to it. Add wheels to the case--they don’t have to roll-and decorate it to look like a coal car or passenger car.
Build a bookcase that looks like a stack of building blocks. Use storage cubes so that you can rearrange them occasionally to keep things interesting. Paint the blocks different colors, and use them to store toys and stuffed animals as well as books.
Bookshelves can be more than just a box setting against a wall.
You can set a tall, narrow bookcase on a heavy-duty Lazy Susan, with some storage on all sides.
Because many kids’ books are narrow, shelves can be very short. Create odd patterns of shelves at different levels, perhaps positioned against a picture painted on a wall. For example, an octopus could hold eight different shelves, one with each tentacle. Let shelves run along tree branches, or let them be cars driving down mountain roads, or boats floating down river. The shelves don't have to be perfectly flat to do their job; let the shelves be “tossed around” on falling leaves.
If many of your child’s books are on the same subject, make a bookcase specially designed for the topic. Put horse books in a corral bookcase, or boat books on a shelf with cutout waves covering the front edge of the shelves. Your only limit is your imagination.