The Boy in the Box
An unidentified Caucasian male, probable age 4 to 6 years, whose nude body, wrapped in a cheap flannel blanket, was found lying face up inside a large cardboard carton just a few feet from the edge of Susquehanna Road .-
http://americasunknownchild.net/
Baby Hope
On July 23, 1991, Baby Hope was found on an embankment of the Henry Hudson parkway, just south of the Dyckman toll plaza in Inwood Hill Park, at the northernmost tip of Manhattan. Judging by the remains, the child was extremely malnourished and weighed only about 25 pounds. Police had estimated that she had been four to four-and-a-half years old when she died.
http://stillunsolved.wordpress.com/2012/09/29/henry-hudson-parkway-unidentified-child-baby-hope/
Dennis Martin
7y/o lost in the Great Smoky Mountains National Park, in 1969.
http://www.knoxnews.com/news/2009/jun/28/missing-dennis-martin/
Jacob Wetterling
He was with his brother and another friend when he was abducted at gunpoint by an unknown individual.
http://www.charleyproject.org/cases/w/wetterling_jacob.html
And many others, some already mentioned, some not -
Baby Gabriel Johnson
Sheila & Katherine Lyon
Susan Powell
Ayla Reynolds
Lindsey Baum
Adji Desir
Madeleine McCann
Trenton Duckett
Hailey Dunn
Lisa Irwin
Sabrina Aisenberg
JonBenet Ramsey
Haleigh Cummings
Sadly, this list could go on and on.