AS3 - accessing MovieClip sprites on frame n?

I have a MovieClip object on the stage called mcTest with an associated class “TestMC”. mcTest has 5 frames with a different named Sprite on each one of the frames, let’s call them s1, s2, s3, s4, s5.

In the class file for TestMC, I declare:
public var s1:Sprite;

public var s5:Sprite;

If I don’t declare them, I, of course, get a “can’t create displayobject” error when I play the movieclip. But when I do declare them, their value is null, even if I do this code in the TestMC constructor:

public function TestMC()
  trace("sc3:"+sc3); // prints: "sc3:null"

I need to save and store the coordinates (x,y) of sc3 in the constructor, and then removeChild(sc3). Ideas?

Similarly, is it possible to addChild() to a specific frame/key frame rather than addChild to the entire timeline of the MovieClip?