I just read a kids book, which I'm not going to name and shame - I should review it as I was given it, but I don't feel qualified to do a full review of middle grade.
I can only assume the author, editors, and publisher assumed the target audience would not notice - but there is basically no world. There's no feeling that anything exists outside the story that's presented on the page (not all of which makes sense).
You can get away with that to an extent in a short story, but in a longer work - you have to build your world. Even if you choose to be lazier and write in a world closer to our own (supers, urban fantasy or contemporary/near future science fiction, you still need to give the reader that feel of a larger canvas they don't get to see.
All it takes to show this is casual mentions. Chinese colonies on Mars. In one of my (unpublished) stories I have the MC think about lands to the south where "people are burned black by the sun" - this used to be what white people believed about black people before we got a better understanding of genetics. If a bit of sun turns somebody brown, then obviously it's just that the sun is so intense they've all been burned black. That line immediately tells you that there's a world outside the country the MC lives in, that there are black people in the world somewhere, the level of scientific knowledge they have about the world.
You don't have to know everything, especially if you're writing a single short story, but you need to give the reader that sense of not being told everything the author knows, those little hints about what might be going on off the page and between the adventures.
Otherwise, your story will feel hollow. I'm pretty sure kids notice that as much as adults.