Plot is a character fighting obstacles to achieve a goal.
The Adults Problem (I am making up my own technical term here) is a problem in writing middle grade / middle reader fiction (herafter, MG). In MG, you want your protagonist to be a kid. And you want your protagonist to face and solve his own problems.
Which means that your kid either has to have no adults around (he’s an orphan, he’s lost, he’s at boarding school), or the adults are the badguys or the adults are neutralized in some other way (they are prisoners, they can only help sporadically, they don’t know about the problem).
Below is an excerpt from my current project. The kid protagonist travels with a couple of adults, but in particular with a Troll, Grim Grumblesson. This is the Adults Problem, exacerbated; Grim is a big, powerful adult. I deal with the Adults Problem in various ways, including this snippet, in which I make Grim Grumblesson a liability.
The heroes are exiting a burning hat factory via a rooftop window.