The easy explanation is that these are rites of passage stories, tales of how boys become men, and girls become women. Every society has its rituals by which children prove their worthiness for the adult world. But that does not explain why children in these stories show a whole-hearted bravery adults rarely match, accept burdens too big for them that adults would cringe from. There is also a persistence in children's hold on an idea. The same intentness of focus that can produce bratty continuous demands for what they want, no matter what threats are administered, also allows them to persist in the right against the powerful.
Children also have the protection of innocence, and even ignorance.
The government of Orson Scott Card's Ender's Game uses children who think they are playing video games to save their world and destroy another. That they are being ruthlessly used and discarded adds another dimension to the story. Childish innocence includes a blithe sense of invulnerability, a lack of understanding that death is real and permanent and applies to them too. Adults try to ward off evil with good sense. We buy insurance.