In the context of children's books, a source file refers to the original digital document that contains all the content and layout elements of a book. This can include text, illustrations, graphics, and formatting specifications. These files are crucial during the publishing process as they serve as the primary resource for typesetters, designers, and printers. Maintaining a well-organized source file ensures that all revisions and updates can be easily made, preserving the integrity of the book’s design and content.