Suppose, Container is your parent movieClip and it has many children then Now your target and currentTarget property depends on which object the listener is attached and how much you have nested your movieclips. in my case the Container is your parent movieClip having two or more children.
Container.addEventListener(MouseEvent.MOUSE_DOWN, pickUp);
public function pickUp(param1:MouseEvent) : void
{
var objectToDrag = param1.target;
Container.setChildIndex(param1.target,Container.numChildren-1);
objectToDrag.startDrag(true);
}