This means the adults, I thought, had to be physically absent, evil, or
But, it occurs to them that there's a fourth option, albeit it somewhat limited: ignorance. Which often overlaps. An adult who dismisses a story from a child who has always been reliable in the past, or who has evidence, is evil or incompetent. But the child may have no evidence, the story may be so wild that imagination would be indeed be more plausible, the child may have a long history as a liar. . . .
Magical or other enforcement of silence on the child is another option. And the effect of physical absence can be introduced by having the adults being unable to keep the secret -- perhaps they are questioned under magical truth-telling, or perhaps the possibility is, however distance, a real enough problem that only children, deemed trivial, can act. (In which case, physical absence is better for the child, who doesn't have to actively hide every dangerous clue.)
2But while the set-up requires finesse and other world-building conditions, ignorance is a complete excuse for inaction.