container.getChildAt(index:int):DisplayObject

package ru.testing{

        import flash.display.*;

        import flash.events.*;

        public class Script extends MovieClip {

 

                public function Script () {

                        MovieClip(this.getChildAt(0)).addEventListener(MouseEvent.MOUSE_OVER, onMouseOver);

                }

                 public function onMouseOver (MouseEvent):void {

                        MovieClip(this.getChildAt(0)).gotoAndPlay(2);

                }

        }

}

public function onMouseOver (MouseEvent):void {

   MovieClip(this.getChildAt(0)).gotoAndPlay(2);

}