well, these don't match the description you gave but they're the kid:Cas stories I know, so FWIW:

The Littlest Hunter by bluebells (7/7)
http://archiveofourown.org/users/bluebells/works?fandom_id=27&page=3
» Fandom: Supernatural
» Warnings: De-ageing!
» Pairing(s)/Characters: wee!Castiel, Winchesters
» Summary: As punishment for disobeying, Castiel is sent back in time in the form of a young child - only to be unofficially adopted by John Winchester.


Star Catcher
Rating: PG
Characters: Dean, Castiel, Sam
Genre: Friendship
Summary: A time-traveling mishap will make two children and a fledgling realize that the unexpected can sometimes be a good thing.
http://murkyfragments.livejournal.com/10454.html


Coming Down on a Sunny Day by maychorian 2010bb
Author name: maychorian
Artist name: samibee
Genre: Gen (with minor het in the second half)
Pairing: Jimmy/Amelia, Dean/OFC
Rating: PG-13
Word count: ~34,000
Warnings/Spoilers: (skip) Allusions to child abuse that may be triggery, OC character death, minors in peril, somewhat pro-church/religion, scary imagery
Summary: In 2009, Castiel watches the Apocalypse end disastrously and makes a decision. In 1984, John Winchester suddenly finds himself with another little boy on his hands, one with dark, messy hair and sad blue eyes who won't leave John and his sons alone. And so Sam and Dean get a weirdo big brother. Because their lives aren't complicated enough.
http://www.fanfiction.net/s/6046340/1/Coming_Down_on_a_Sunny_Day


(if you choose to, FFdot N fic can be downloaded in single files in several formats at flagfic dot com