It's the plot all children's fiction must rely upon, because children's fiction must have child (or teenage) protagonists in order for its target market to enjoy it. And those protagonists must somehow become involved in an otherwise adult plot, even though they're not (adults, that is). It's the plot of Spy Kids, Goonies, Parent Trap, and every single Home Alone movie.
It's not always bad, but it can be done badly. It all depends on why the kids have to save the world instead of their parents.
LESSER REASONS
- Adults are too dumb, disinterested, or just plain grown up to believe the truth. The spunky kids, then, must deal with the problem on their own, often with the parents working against them. I'm sure kids love this trope, but I can't stand it. It's insulting, sure, but it's also unrealistic and teaches kids unreasonable amounts of disrespect (says the guy who's had to parent kids raised on this trope). Examples: Home Alone, Parent Trap, Lost Boys, Goonies.
- Adults are incapacitated or otherwise out of the picture, and the spunky kids must fill their shoes, even though they're totally unqualified. It's even worse if qualified adults exist, but are too dumb, disinterested, etc. to get involved. Example: Spy Kids.
- Child of prophecy. The adults cannot save the world because it is not their destiny--even though they're more qualified. Example: The Sword in the Stone; The Lion, the Witch, and the Wardrobe; The Hobbit; Lord of the Rings; Mighty Morphin' Power Rangers.
BETTER REASONS
- Supernatural or unnatural ability. The spunky kid is actually capable of something nobody else can do. Their ability just happens to be needed before they have a chance to grow into adulthood. Examples: Harry Potter, Ender's Game.
- Adults are incapacitated or otherwise out of the picture, but qualified adults don't exist, and the kids manage to win in spite of their shortcomings. Examples: Eragon, Star Wars.
- Adults cannot be involved, either because the rules of the conflict don't allow it, or because the spunky kids will fail if the adults know. Examples: Jumanji, Zathura, Adventures in Babysitting.
Anyway, that's just my opinion. How do you feel about this trope? Where have you seen it done well? Done poorly?