put a “back” button in front on an embed map

Hi all,

I've integrate a map from MapQuest in my Adobe Air app (in AS3).

The map is taking all the screen, as I want.

BUT, I'd like to add a "back" button in order to go back to the previous menu.

Here's my code :

var NoumeaNord:TileMap = new TileMap("KEYcode");


            //set the size of the map
            NoumeaNord.size = new Size(800, 533);


            //add the map to the sprite.
            addChild(NoumeaNord);


              NoumeaNord.addControl(new SMLargeZoomControl());
              NoumeaNord.addControl(new MouseWheelZoomControl());
              NoumeaNord.addShape(new Poi(new      LatLng(-22.2758000,166.4580000)));
              NoumeaNord.setCenter(new LatLng(-22.2758000,166.4580000),15);




         function addBackBtn():void{


            var back:MovieClip;
                    back = new backBtn
            addChild(back);
            back.x = 0;
            back.y = 400;
            setChildIndex(back,0);
}

Don't know why but the BackBtn won't be in front of the map ! I've tried with setChildIndex(back,-1); but it makes an error : "RangeError: Error #2006: index is off limit".

Any idea ?