I saw a news video in which an "alleged aunt" of the white girl's baby is a black woman, speaking to a reporter with her back turned to the camera, but her black face is briefly visible near the end of the video. So there's a possibility that the girl's baby was mixed race.
http://www.kare11.com/video/1871390380001/1/Teen-admits-to-strangling-newborn-putting-in-shoebox
http://www.theledger.com/article/20120928/NEWS/120929363
http://wusfnews.wusf.usf.edu/post/polk-deputies-charge-14-year-old-homicide-death-her-newborn-son
http://news.gather.com/viewArticle.action?articleId=281474981667494