It's a shame that containers can't be used for this.
Anyways, basically what you want to do is to give Object C a value called "attached". When Object C is created, set the "attached" value to the UID of Object A. Then, to pick it when you mouse over Object A, do something like this:
(Both of these are conditions in the same event)
+Mouse is over Object A
+Object C: Value 'attached' is equal to Object A.UID
-Object A: Set Opacity to X
-Object C: Set Opacity to X
I hope I explained that clearly enough.