In your example code, you have created a bunch of button instances, and one by one you have added them to the display list of flagDisplayButtons. They are on that display list in the order that you placed them.
first instance is at index 0
second instance is at index 1
third instance is at index 2
and so on.
So if you want to access the third button you added, you could do this :
var flagButton:RoundButton = flagDisplayButtons.getChildAt(2) as RoundButton;
Now that you have created variable to reference that button, you can do any of these :
flagButton.alpha = .5;
flagButton.x = 100;
flagButton.y = 200;
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