Child labor is inhibiting to a child's growth
Children's education would be inhibited
Children would often drop out of school so they could work (often in factories). Some would miss a lot of school, while many quit school completely and never went back.
Too long of hours
Hours were often way too long for adults to handle, let alone children. Children would sometimes work 19 hours a day, but usually they worked 12 to 14 hours, with little break. With these hours children would not be able to get enough sleep that a child needs (12 hours) let alone have any time to play and allow their imagination to develop. Not only would they receive little pay, but children would often start working at age six.
Working was dangerous
Accidents that resulted in permanent injury and even death would often happen at factories. Unguarded machinery was an issue as one hospital even reported thousands each year with factory related injury and loss of limbs. We shouldn't let a child so young receive an accident such as chopping their hands off that would ruin them for life at such a young age. Not only would that happen but injuries would occur as a result of abuse by employers frequently.