Maximize storage with built-ins. In a space that will be filled with toys, books, sheets, blankets and more, adequate storage is key. "Consider building closed cabinets in lower units approximately 24 inches deep,” advises Rosen. “Twelve-inch bookcases with adjustable shelves should ideally rest on top of the lower cabinets, allowing enough space to place accessories. Remember, fill the bookcases with books for the kids as well as for guests.”