I've read a great deal about slavery in the south. Aside from the obvious issue of being owned, being fed, housed,and clothed is not exactly no compensation.
Children were employed because they were the indigenous 3rd world. They routinely put in 72 hr work weeks.
The owner carried a whip how different is that from the slave situation.
"Slavery" isn't about quality of life. It's about being forced (by the threat of violence) into servitude. Most of our forefathers lived a harsh existence. But, a harsh existence does not make slavery. Slavery is about not being permitted to make choices for yourself.
Consider what we're facing today: Suppose the president mandates community service for young people, senior citizens, anyone... The conditions might not be nearly as harsh as the conditions slaves and child laborers in the past faced. But mandating community service would be forced labor (slavery).
To put it another way: If the government begins requiring by law people to sweep the streets, that would be slavery/forced labor. But, if a person willingly accepts a job sweeping the streets, that's not slavery.
A child laborer was either sent to a factory willingly by his parents (or he seeked employment on his own) so that he could help earn money for his family. But, the factory owner wasn't forcing him to be there. One could make the argument that parents treated their children like slaves, but that's a different discussion.
In cases where children are/were held as slaves and forced to work in factories - THAT was slavery/forced labor. But, that wasn't the case most of the time, according to what I've read and was told. (Maybe you've read/heard something different.)