Why was the use of child workers accepted and even encouraged in the nineteenth

At the time, children were the cheapest source of labor, and this therefore cut down operating costs. There wasn't really anything like modern child protective services, and children were unlikely to advocate for fair wages or treatment, as they just did whatever their parents told them, and parents desperately needed the additional source of income. Also, many industries of the nineteenth century, probably most famously textile mills, liked hiring children because their small fingers could reach into small crevices in machinery (to untangle thread, for ex) easier than adult hands.

In the early days of industrialization, an "anything goes" philosophy was popular, permitting capitalists to abuse workers and subject them to unsafe working conditions. This included children. Wages were so low that everyone in the family worked (if possible) to bring additional pennies into the family income. As technology progressed, children were no longer needed to do menial work and society’s philosophies about the treatment of children greatly changed. It is unlikely that children in the Least Industrialized Nations will have the same reprieve as American children did. The Least Industrialized Nations do not have the manufacturing and technology base that evolved in American society. These nations have been engaging in the same type of abuse for hundreds of years. There is no indication that the type of work in which children are abused will change and as a result, young, strong boys (an in some cases girls) will be needed to support these primitive economies.