Sadly, there were thousands, some even younger than Carmela.
When she was a baby, Carmela Teoli and her family moved from Italy to Lawrence, Massachusetts, to join her father who was already living there. She began working in the Washington Mill some time after turning thirteen (although reports on her age vary.) She really was injured on the job, and her story really did help gain the strikers much-needed public support for their cause. And yes, she actually did testify before a Congressional subcommitee. You can read more about her in Frank Palumbo Jr.'s book Through Carmela's Eyes, available at Amazon.
Children worked at mills throughout the country, not just in the northeast. Young girls, as well as boys, of course, labored in Massachusetts, the Carolinas, Texas, and in many other states. This photo of Fannie, age 7, was taken by Lewis Hines, a man who made it his mission to expose underage child labor and dangerous working conditions for young people throughout the early 1900s. You can view hundreds more such photos at the Library of Congress.