Object3D has a function called setTransparency(int)... I was recommending that you create your selection aura as a plane with some transparency on it and then addChild from the selected parent.
You can also do something like the car sample to place the circle properly on the ground. I believe that the car sample has 4 surrounding children that help calculate the angle and height where the parent should be, which is the selection aura in your case. If your level is all flat, then maybe the selection aura can figure out how high it should be placed; I was mentioning the car sample's 4 children because that demo has hills and so on...
I hope this helps!