This is a protest poem decrying the horror of child labor in factories and coal mines of England.
Children were put out to work at very young ages in factories and in the coal mines. They were small enough to fit in the tight spaces of the mines and used to pull carts of coal up from the depths. Many children died from brown lung disease and other diseases of overwork, malnutrition and tuberculosis during this time period.
Children did not have the "childhood" experiences that we modern people associate with "childhood" unless they were born in the upper wealthy class. Most children were put out to factory work by the time they were 8 or 9 years old. Children began working in the coal mines at around the same ages.
Child labor laws were not yet common practice, so any child of any age could be put out to work. Most factories worked 13-15 hours, and there was no such thing as a break from work. The work was dangerous as well because there were no safety regulations in the factories or mines. Browning's poem is a protest poem which did have a large impact on the readers of her poetry.