RangeError: Error #2006: The supplied index is out of bounds

Hi Guys
I am making an Auto-Arrange button and clicking on which arranges all the children of the container( which are on the main stage) in the tile format. Basically on clicking, I manually move those child on the stage to particular coordinates as shown in my code. I have pasted my code below. My problem is that as soon as i click on the auto arrange button I am getting error “RangeError: Error #2006: The supplied index is out of bounds.”. Please let me know what i am doing wrong and i will appreciate if you let me know how would i fix this
Thanks a lot guys
Anuj

CODE****
var aa:Number=0;
var xcoord:Number=-300;
var ycoord:Number=-200;
var xcoord1:Number=450;
var xcoord2:Number=850;
var xcoord3:Number=1250;
var ycoord1:Number=350;
var ycoord2:Number=650;
//Button Listener
btn_AA.addEventListener(MouseEvent.CLICK,autoArrange);
function autoArrange(event:MouseEvent):void
{
if(container.getChildAt(aa)!=null)
{
container.getChildAt(aa).x=xcoord;
container.getChildAt(aa).y=ycoord;
//Arrange Second
container.getChildAt(aa+1).x=xcoord+xcoord1;
container.getChildAt(aa+1).y=ycoord;
//Arrange Third
container.getChildAt(aa+2).x=xcoord+ xcoord2;
container.getChildAt(aa+2).y=ycoord;
//Arrange Forth
container.getChildAt(aa+3).x=xcoord+xcoord3;
container.getChildAt(aa+3).y=ycoord;
//Arrange Fifth
container.getChildAt(aa+4).x=xcoord;
container.getChildAt(aa+4).y=ycoord+ycoord1;
//Arrange Sixth
container.getChildAt(aa+5).x=xcoord+xcoord1;
container.getChildAt(aa+5).y=ycoord+ycoord1;
//Arrange Seventh
container.getChildAt(aa+6).x=xcoord+ xcoord2;
container.getChildAt(aa+6).y=ycoord+ycoord1;
//Arrange Eight
container.getChildAt(aa+7).x=xcoord+xcoord3;
container.getChildAt(aa+7).y=ycoord+ycoord1;
//Arrange Ninth
container.getChildAt(aa+8).x=xcoord;
container.getChildAt(aa+8).y=ycoord+ycoord2;
//Arrange Tenth
container.getChildAt(aa+9).x=xcoord+xcoord1;
container.getChildAt(aa+9).y=ycoord+ycoord2;
//Arrange Eleventh
container.getChildAt(aa+10).x=xcoord+ xcoord2;
container.getChildAt(aa+10).y=ycoord+ycoord2;
}
}