Why wouldn't you include the error messages you're getting?

I'll guess that you are loading the swf with a Loader and it's still a child of that loader. You could add it to the parent swf directly in a complete handler for that loader, or you could use parent.parent. But what you should really do is have a function in the loaded swf that takes the value you want as a parameter. Then call that function from the parent swf when you want to. Or you could have a function that saves a local variable with the value and call that in the complete handler.

Let's say you want to keep a value called "foo". You've retrieved this value in the parent somehow, it doesn't matter for this exercise.

loaded swf has code more or less like this:
Code:
var foo:String; //loaded swf's foo variable

function setFoo(f:String):void{
  foo = f;
}
and the loading swf has code something like this:
Code:
var loader:Loader = new Loader();
loader.contentLoaderInfo.addEventListener(Event.COMPLETE, handleComplete);
loader.load(new URLRequest("myChild.swf"));

function handleComplete(e:Event):void{
  var l:Loader = LoaderInfo(e.currentTarget);
  var clip:MovieClip = MovieClip(l.content);
  clip.setFoo(foo);
}