There's a huge collection of these at the Library of Congress - the National Child Labor Committee Collection:
Founded in 1904, the National Child Labor Committee set out on a mission of "promoting the rights, awareness, dignity, well-being and education of children and youth as they relate to work and working." Starting in 1908, the Committee hired Lewis W. Hine (1874-1940), first on a temporary and then on a permanent basis, to carry out investigative and photographic work for the organization.The book Kids at Work: Lewis Hine and the Crusade Against Child Labor is a photobiography of Lewis Hine, using his own work as illustrations. Related books available here.
|Some of the young cartoners in work room, Seacoast Canning Co., Factory #2. Five year old Preston working also. Location: Eastport, Maine.|
"Boy Wanted" Sign. West 19[th] Street. Location: New York, New York (State) / L.W. Hine.
|Night Shift Leaving for Home: Indiana Glass Works, 8:00 A. M., (See also Photo #91.) Location: Indiana.|
|Name: Norris Lovitt. Been picking for 3 years in berry fields near Baltimore, Md. July 8, 1909. Location: Baltimore, Maryland.|
|Callie Campbell, 11 years old, picks 75 to 125 pounds of cotton a day, and totes 50 pounds of it when sack gets full. "No, I don't like it very much." Location: [Potawotamie County, Oklahoma].|
|9 year old girl carrying garments down Blackstone Street, Boston, Mass., to a Hanover Street home. She finishes 8 pairs of trousers a day and gets 8 cents a piece. Location: Boston, Massachusetts.|