Find a good secondary source, and work backwards. See what primary sources the author consulted, and search for them (you will probably find other relevant sources this way).
Think like an archivist. In archives, sources aren't organized by subject as they are in the library, but by the person, organization or group that created them. Start from what you know -- names of people, organizations, schools, or programs -- and do an internet search using that name plus a word like "papers" or "collection."
Primary sources are original, uninterpreted information, such as firsthand accounts of events in letters, diaries, interviews, or historical news reportage.
Key Sources - All Eras
19th Century U.S.
20th Century U.S.
Children & Youth in History offers sources about young people from around the world & across centuries, with a tremendously helpful guide to understanding & interpreting primary sources on children & youth.