Shifting ideas about the qualities of children's imaginations transformed relationships between adults and children in nineteenth-century Britain. This dissertation contends that these new paradigms of children's fancy led authors of children's literature to partner with the young as creative collaborators, which accounts for frequent representations of children as an adult author's auditor, coauthor, illustrator, or guiding genius. These intergenerational collaborations were new models of authorship and evidence of a growing cultural imperative to recognize the young as active agents shaping their own social worlds. Alert to the fact that depictions of children are historically variable, I situate children's literature with and against discourses from psychology to education reform, demonstrating how the perceived powers of fancy granted children agency in a variety of cultural arenas. My project, then, offers an alternative to critical accounts that represent children as ciphers fulfilling adults' psychological and sexual desires. My introduction examines children's literature of the early nineteenth century, which I contend was a collaboration between adults Debates about the child's imagination, however, indicate a shift in expectations regarding adults' relationships to children. The remaining chapters detail the consequences of this shift, exploring four ways children were acknowledged as creative collaborators. Chapter one explores how many authors for children, inspired by fairy tale collections and cultural associations between children and preliterate cultures, structured their fictions according to models of oral narration. These authors defined children not as silent listeners but as participants in the narrative. Chapter two investigates coauthorship in the work of Robert Louis Stevenson, who understood composition as a collaboration between multiple familial, literary, and psychological personas. Partnering with his stepson, Stevenson developed a vocabulary of images that resurface throughout his works and express a social model of authorship. My third chapter explores the unruly child, examining children's literature that depicts collaborations between disobedient children and dim-witted adults in the context of education reforms that privileged imagination over adult authority. The figure of the disorderly child suggests anxieties about the imaginative power of those considered socially vulnerable. I conclude with a chapter on illustration, situating images by Edward Lear and Rudyard Kipling against ideas about children and art, arguing that these author-illustrators fuse childlike spontaneity and adult order, representing collaboration through playful images.