I am not sure why you can’t do select child actors from the world outliner. I just tried it myself and it doesn’t seem to work. You can, however, as an alternative, double-click the parent actor in the world outliner to focus on it and then, in the viewport, double-click the child actor to select it. That will work too.
As for double-clicking the actor in the world outliner, I am not sure when or if they will fix that for real.