Might be:
- As Long As I'm the Hero of This Little Girl (http://notthequiettype.livejournal.com/261169.html) by notthequiettype, which is a post-apocalypse fic in which Sam and Dean found a baby girl but didn't fight...
or:
- estrella30's babyverse (http://estrella30.livejournal.com/tag/oh+god+it's+babyfic) by estrella30, which isn't post-apocalypse but where Dean and Sam found a girl and had an argument about the color of the clothes Dean bought for her in the one where they decide to keep her (http://estrella30.livejournal.com/337016.html)