I’m no lit major, but I love this song too, so here’s my stab at analysis…
IMO the song as a whole is a mid-life reflection on a life rather easily, comfortably and thus somewhat quickly lived through.
Sonny is “accustomed to a smooth ride” – born to well-off parents and grew up happily enough, got a job and a family, etc., everything going according to plan and without a hitch. But now he finds that he can’t sleep through the night, as he feels age creeping up on him (feeling like “a dog that’s lost its bite”, with thinning brown hair), realizes that people he grew up with are starting to die off, and thinks about the meaning of it all (“wandering beyond his interior walls”).
His life so far has been a happy one, he really can’t complain, there are plenty who have it much worse. But he feels somehow constricted, like it’s all been lived within prescribed boundaries on a path set out for him. How it’s strange that some rooms are like cages… And is the only way out of this “room”, to walk through the shadowy door of death? Can one grasp for something more or different? (This is the classic “mid-life crisis”)
In this context “the obvious child” has religious or existential overtones, as a contradiction of the statement “the sky is just the sky”. To a child all things are magical, even things as mundane as looking up at the daytime or night sky. If it’s obvious to a child… Why deny it as an adult, especially when that magic would be so comforting?
As for “the cross is in the ballpark”, I always thought of it as “in the ballpark” meaning “in play” (as in baseball, for a ball that is hit but not out of the ballpark) and “the cross” would be an image of religious enlightenment.