Well, I found a Guy Clark song recorded by Jerry Jeff Walker. I'll betcha that's it. I love Guy Clark's songs. I won't vouch for the accuracy of these lyrics.
-Joe-

^^ The Cape
Guy Clark (as recorded by Jerry Jeff Walker)

Well he's eight years old, he's got a flour sack cape tied all around his neck
He's climbing up on top the garage, figurin' what the heck
He screws his courage up so darn tight, that the whole thing come unwound
He got a runnin' start and bless his heart, he headed for the ground

Chorus
'Cause he's one of those who knows his life
Is just a leap of faith
Gotta' spread your arms and hold your breath
And always trust your cape

Well he's all grown up, he's got a flour sack cape tied all around his dream
He's full of piss and vinegar, ah, he's bustin' at the seams
He licks his finger and checks the wind, it's gonna be do or die
He wasn't scared of nothin', boys, he was pretty sure he could fly
Chorus

Now he's old and grey, with a flour sack cape tied all around his head
And he's still jumpin' off the garage and will be til he's dead
All these years the people said the fool was actin' like a kid
And since he didn't know he couldn't fly, well of course he did
Chorus


source: http://lyrics.duble.com/lyrics/J/jerry-jeff-walker-lyrics/jerry-jeff-walker-the-cape-lyrics.htm


Reminds me of my stepson, skulking around the rooftop in a batman cape and suit he made. I hope he never loses sight of his dreams.