Complex sentence: In 1800 there were 20,000 apprentices working in cotton mills and the enclosed conditions and close contact within factories caused diseases to spread rapidly leading public opinion to suggest that working conditions needed to improve for children.
Split into simpler sentences: In 1800 there were 20,000 apprentices working in cotton mills and the close contact and enclosed conditions within factories caused the diseases to spread rapidly. This led to a growing public opinion that working conditions needed to improve for children.