I looked through NamUs for kids aged 8 - 14, missing within the past 5 years from states neighboring Missouri. 9 people came up. I've also included the amount of time it'd take to get from the place they went missing to where the UID was found.
ILLINOIS
Mateo Elizar - Missing since 2-4-2011 from Rantoul. Distance is 4h and 55min
Emily Bracamontes Missing since 4-08-2011 from Rockford. Distance is 6h and 25 min
IOWA
Michael McGlothlin Missing since 12-11-2011 from Des Moines. Distance is 5h and 35min
KENTUCKY
Citlalli Perex-Coronel Missing since 10-05-2012 from Louisville. Distance is 6h and 50min
TENNESSEE
Chloie Leverette and
http://www.charleyproject.org/cases/d/daniel_christopher.html Missing since 10-23-2012 from Unionville. Distance is 7h and 25min. I doubt the UID is either of them, though, since only one skeleton was found and authorities in Chloie and Christopher's case say that foul play is not suspected (the house burned down and it's possible that their remains just weren't found in the wreckage).
Isaac Savoie Missing since 8-11-2009 from Franklin. Distance is 6h and 55min
ARKANSAS
None.
OKLAHOMA
Marianna Peevy Missing since 9-20-2012 from Oklahoma City. Distance is 5h and 35min
KANSAS
Jamie Zubia-Hernandez Missing since 6-9-2012 from Wathena OR Kansas City. Distance from Wathena is 4h and 15min, distance from Kansas City is 3h and 25min