I think that what makes a story effective. When a story, any story, dark or light, is about a child, we instantly sense the innocence, the wonder! Everything to a child is NEW and EXCITING.
Even when the story is dark, there is an element of optimism and determination that only a child has because the morals are so black and white. Stranger Things and Pans Labyrinth is super effective on contrasting the mindset of the children with that of the adults. The Party in Strange Things consists of pre-teens who play DnD and thus relate everything around them to a Fantasy Quest. This contrasts with the terror felt by the older teens and the adults when kids are going missing or found dead and the police are trying to solve a mystery. We contrast the Fantasy Quest adventure taken seriously by the kids with the very heavy stakes carried by the adults.
Note that while the main party survive mostly (mostly) unscathed, multiple people, young and old, do die throughout all three seasons. Season 3 especially ups the stakes and it's pretty freaking graphic.
In Pans Labyrinth, the little girl is trying to solve a Fairy Tale quest (maybe real, maybe not) in order to become a princess and reunite with her 'real' family. She is also juggling saving her mom and then later, faces a true lesson in selflessness with her baby brother. The black and white fable of her quest is contrasted very heavily and effectively with the very real horror of WW2 and a sadistic colonel (general? can't remember) who tortures rebels and treats his heavily pregnant wife more like an incubator than a human being. While we can definitely say that he is the BAD BAD guy, the rebels do make questionable decisions a child can't really comprehend and some people on the BAD side of the war are not monsters like the evil evil colonel.
Oh and Spoilers
The MC, the little girl, dies on screen
These are effective and strong characters who are definitely not just fodder, or just exist for the author to torment relentlessy (Justine by Marquis De Sade, anyone?). Honestly, I'm happy to see more stories, even dark stories, starring children. Why?
Because in the face of ultimate darkness, children shine brightly and show hope. We want to protect that hope, even as those kids run head-first into danger.