I believe that the main reason it was so dangerous was the limitations on health and safety regulations enforcing employers to properly care for their staff.
Children were allowed to be over worked and underpaid.
Work conditions were dangerous, often with poor ventilation and disregard for proper safety measures when operating or working on machinery.
Oftentimes, any safety regulations that might have been followed were often ignored when it came to immigrants.
Most of the time workers, children included, worked long hours well into the night and would suffer from the possibility of fatigue and malnourishment.
I hope this helps some!