Access a child of a child's stage?

Hi All,
I am a newbe to AS3 so pls be patient.

I have a mainSwf in which I load an external swf, which loads an external swf.
( a child of a child). I am trying to set the framerate via AS3 for the childs child by using stage.frameRate= 15;

It doesnt work when the children are loaded into the mainSwf. Any Idea how to access the stage of a child’s child?
I have tried root, Movieclip, this but can not get it to work

Thankfull for all help

Many thanx