England saw a surge in population at the time, outpacing the food supply. That is the explanation for the drop in life expectancy. People did not migrate from the farms to the factories in order to lower their standard of living.
There were plenty of examples in Factory Act testimonies by inspectors saying air quality for children in factories was unbearable leading to lung problems later on in life (average death being under 40)
People wouldn't have sent their kids to the factories if life was better on the farm. I doubt that many people at the time were aware of the long term effects of smoke inhalation.
My understanding is that the UK experienced a population boom due to increased farming productivity due to potatoes. The resulting population boom meant that there where more farmers than farms, so they had to go somewhere, and the only option was cities.