Why “Just Believe”?
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. … Why does this theme occur so often in children’s fiction? … Perhaps religious and political leaders, among others, would like to see youth raised to believe without question. … 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.
David Friedman suggests:
An alternative explanation is that adults believe, with some justice, that they know more than children. In their interaction with children, they find themselves in the situation of telling children things the adults are sure are true but either cannot persuade the children of or are not willing to take the trouble to persuade the children of. … Hence the attraction–to adult authors and adult purchasers of children’s books–of scenarios where the wise person representing the adult is telling the younger and less wise person representing the child to "just believe."
These explanations don’t ring true to me. Instead, I suspect we know our children better gain allies by seeming innocent and trusting.
P.S. My mother writes Christian tween girl fiction, and today is my parents’ fiftieth wedding anniversary.
The content of fiction is largely market-driven - especially if we limit ourselves to looking at fiction that has already been filtered out by market success. Thus we might expect that the reason a lot of fiction promotes credulity, is that children want to believe. They are not the only ones who want to believe. Religions are popular among adults.
Maybe exhorting children is compulsive behavior for adults, and "Believe!" seems innocuous enough that it can be thrown into stories that don't actually have a moral point.
Afaik, Hans Christian Anderson's stories don't have "Believe!" in them, and I suspect exhortions to believe is a relatively modern thing, possibly a side effect of religious tolerance. There's still a background belief that religion is a good thing, while it wouldn't be welcome to promote any particular religion to a general audience.