Children's fiction often promotes credulity as a virtue. Consider, for instance, the admonitions in Disney's Peter Pan, in Elf, or in The Neverending Story. These and many other works teach our children, "Just believe!"
Children's fiction employs this trope so often that it fits a formula. A wise character tries to convince the protagonist that something wonderful will happen if only he or she will earnestly believe an improbability. Consider, for instance, how Yoda tells Luke to cast aside all doubt if he wants to levitate his x-wing from the swamps of Dagobah. "Do, or do not. There is no try," Yoda explains. Following the usual script, Luke resists, courting disaster, before he finally embraces faith and wins its rewards.
Why does this theme occur so often in children's fiction? Such works often aim to instill in children the sorts of virtues that we value in adults, such as bravery and kindness. I don't think that moral instruction adequately explains the extent to which children's fiction promotes credulity, however. Perhaps religious and political leaders, among others, would like to see youth raised to believe without question. But other parties, at least as equally influential in shaping children's fiction, favor the contrary values of independent thinking and rational inquiry.
I propose a different, less conspiratorial cause. I suspect that children's fiction so often promotes gullibility as a virtue because those who author such works know, at some level, that they rely on children's gullibility. Given its counter-factual presumptions—that we can fly, or that Santa Claus exists, or that we can lift heavy objects just by thinking about them—children's fiction requires the willful suspension of disbelief. By arguing against rational skepticism, the authors of such works wear down the defenses that might otherwise deflate the impact of their stories.
I have nothing against entertaining children—or adults!—with fantasy. I do wish, however, that that the authors of such works would stop preaching credulity. The best works of fiction don't require such rhetorical slight-of-hand, as they create worlds so internally consistent and rich that we don't hesistate to buy into them (consider, for instance, the works of Tolkien or Rowling). Only hacks feel the need to teach our kids ignorance.