During the 19th century the presumption that Fairytales and Fantasy stories were forms of literature meant for the nursery led to the natural use of children as protagonists in such tales; moreover, whether or not their protagonists were children, authors of 19th-century texts normally addressed their tales downwards to an imagined audience of young folk. This presumption, and the literary strategies dependent upon it, generated huge amounts of now-unreadable literature.
It is now an axiom in Children's Fantasy – as in any writing for the young – that children should not be patronized, although the protagonists of tales designed to appeal to children are usually children. In fantasy written for adults, by contrast, protagonists are often first encountered as children but normally reach adulthood during their adventures: most fantasy novels are shaped around Quests which involve their Heroes and Heroines undergoing trials, being subjected to Metamorphosis and gaining maturity (> Monomyth), and it is a natural strategy to dramatize the Rite of Passage into full empowerment as the literal growth of a child or adolescent into adulthood.
Non-protagonist children in fantasy are very much less common; they are often killed early, or prove to be Companions whose true nature has been disguised. In Supernatural Fiction, non-protagonist children tend to be victims of Possession, or may be literal Ghosts. In Horror, non-protagonist children are often Evil. [JC]