From the FBI NCIC Missing Person and Unidentified Person Statistics for 2010:
http://www.fbi.gov/about-us/cjis/nci...stics-for-2010
Out of a total of 565,692 missing persons under the age of 18; 314,117 were white. Thats 55%.
For 2012:
http://www.fbi.gov/about-us/cjis/nci...stics-for-2012
Total under 18: 497,327; Whites: 288,899; 58%