Recent studies have highlighted the high net undercount of children, particularly young children, in the 2010 U.S. Decennial Census. However this issue has received little systematic attention from demographers historically. Given the extensive use of decennial census data, the high net undercount of young children is both a data and a social equity problem. This study examines patterns and trends in the net undercount of children in the U.S. Decennial Census from 1950 to 2010. The focus is on trends in the net undercount of children relative to adults. The initial emphasis on all children (age 0 to 17) shifts to a focus on young children (age 0 to 4) where net undercount rates are the higher than any other age group. Differences between net undercount rates of Black and Non-Black populations are also examined over the 1950 to 2010 period. Results show that the differential net undercount of young children relative to adults as well as older children is now larger than the differential net undercount of Blacks and Non-Blacks.