Entry Points and Short Fiction

It's deceptively simple in appearance, but hard in actual practice. Try to write a short story set in
your huge ficional universe, supposedly as a way of paving the road for ultimately getting the whole
series of huge novels published. But as you start out, you keep realizing that you've got to explain this element, or that element. Why does this sort of character do things in a certain way? Why
does a certain kind of technology (or magic) only work in a certain way and is always associated with a certain set of traits?

Complex relationships have been developed over the course of all those novels that are sitting in a
dresser drawer, waiting to be published. Someday. How to present these relationships -- or to somehow pull out one small section of them -- without doing violence to the whole, and quite possibly end up destroying the very novels you're trying to get published in the long term?

Recently I pulled back out a story that I originally wrote and abandoned almost ten years ago. It's set
several decades into my alternate-history universe about human cloning and an alternate fall of the
Soviet Union. In that world, America had its own human-biotechnology project in response to the Soviet one, and now the chickens are coming home to roost. But how can I establish this situation quickly enough so as not to totally overbalance a short story of reasonable length with a huge chunk of backfill at the beginning? And for that matter, how much information do I need to present in order to have the story be comprehensible, and what can I simply pass over as irrelevant to the story at hand?

One factor that I'm hoping to use to simplify the problem is my protagonist's relatively young age. It's
perfectly believable that a child of relatively tender years would be ignorant of the complexities of the
history that led to the world in which she lives. She simply accepts it as That Which Is.

However, a young child has relatively little latitude for action, often not enough to really carry a story. For instance, there are several places where my protagonist really needs to be able to come and go and to meet with people outside her family and school peer group without immediate parental supervision. Thus she needs to be old enough that her parents would believably let her get on a city bus by herself and go to the places she needs to go in the course of the story. But the older she is, the more she will be expected to know and understand about the history of the world she lives in, particularly as it relates to the biotech and the prejudice it has spawned. Round and round I go, in a frustrating circle. Can this story be saved, or is it a lost cause?