You are erring with the assumption about slave labor.
The south had slave labor the north had child labor.
A distinction without a difference.
There were problems with child labor, but there was a major distinction between slave labor and child labor.
Child laborers were not “owned” as human slaves nor forced into factories by business owners. They were sent to those factories by their parents to earn money for their families. (One might argue they were “owned” in a way by their parents, but that would be an entirely different debate/discussion.)
My grandparents started work in factories as young children. When the government came around to check the factory, my grandfather along with the other children were sent outside the factory to hide. But, my grandfather said none of the children wanted the government people to see them, either, because then they would lose their jobs and the money to bring home. Those families needed to put food on the table. Once, my grandfather was fired for grabbing the whip the factory owner snapped at him, and his mother packed up food and cash to bring to the owner to apologize and ask for his job back.
So, it wasn’t a nice situation by today’s standards, but it wasn’t slavery. Just like business owners hiring poorer people in “Third World” countries isn’t slavery, either.