Class Access child on stage

programming flash
1

I need a class code to access a child instance of another class code that is on the stage. I have a class code named Ball.as. On the Time Line is a child of the Thing class. It is call newThing. Here is the code in the Ball.as file:


package {
    import flash.display.MovieClip;
    import flash.events.*;

    public dynamic class Ball extends MovieClip {
        private var _root:Object;
        public function Ball() {
            addEventListener(Event.ADDED, beginClass);
            addEventListener(Event.ENTER_FRAME, eFrame);
        }
        private function beginClass(event:Event):void {
            _root=MovieClip(root);
            this.x=newThing.x;
            this.y=newThing.y;
        }
        private function eFrame(event:Event):void {
            // do stuff
        }
    }
}

I get an error message saying that newThing is an undefined property. Please help me out. I’ve been trying to figure this out for weeks with no luck. Thanks in advance.