I found a few, just going by "pre-teen". Not sure how "pre" that is indicating. Going with the possibility that he/she could have been missing well before they died:
CHRISTIAN MIGUEL SANCHEZ
NCMC1075934
-Family Abduction (View Poster)
DOB: May 12, 2005 Age: 8
Missing: Jul 8, 2007 Race: Hispanic
Location: PERRYVILLE, MO, US
VERONICA NICHOLE SANCHEZ
NCMC1075934
-Family Abduction (View Poster)
DOB: Sep 29, 2006 Age: 7
Missing: Jul 8, 2007 Race: Hispanic
Location: PERRYVILLE, MO, US
EMILY ALINA KOYAMA - although she has been reported out of country
NCMC1115920
-Family Abduction (View Poster)
DOB: Jul 5, 2008 Age: 5
Missing: Feb 2, 2009 Race: Biracial
Location: SPRINGFIELD, MO, US
LISA IRWIN
NCMC1180911
-Endangered Missing (View Poster)
DOB: Nov 11, 2010 Age: 2
Missing: Oct 3, 2011 Race: White
Location: KANSAS CITY, MO, US
ALLYSON CORRALES
NCMC1117576
-Endangered Missing (View Poster)
DOB: Jan 13, 2005 Age: 8
Missing: Mar 6, 2009 Race: White
Location: KANSAS CITY, MO, US
CHLOE COMBE-RIVAS
NCMC1045788
-Family Abduction (View Poster)
DOB: Jul 8, 2003 Age: 10
Missing: Mar 15, 2006 Race: White
Location: KANSAS CITY, MO, US
That's everybody listed as Missouri on NCEC that would currently be less than 13.