[flexcoders] undefined method getChildAt

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I have a VBox in mxml called vidButtonBox
<mx:VBox id="vidButtonBox" height="435" width="215"  
styleName="vidButtonBoxStyle" paddingLeft="10" paddingRight="10"  > 
</mx:VBox>

In action script, I create a VBox with a Vbox child
var vidArea:VBox = new VBox;
var vidButton:VBox=newBbox;
vidArea.addChild(vidButton);

then add that to the mxml Box
vidButtonBox.addChild(vidArea);


Then I try to access the children
var theVidButton:VBox= vidButtonBox.getChildAt(0).getChildAt(0);

And I get this error:
K:\xampp\htdocs\jbh\Main.mxml(145):  Error: Call to a possibly undefined 
method getChildAt through a reference with static type 
flash.display:DisplayObject.

This makes no sense at all. What is going on here ? How do I access 
child[0].child[0]?