Your source does not actually give the data or methodology for their claim that child labour peaked during the industrial revolution, so there is no way to know

Your source does not actually give the data or methodology for their claim that child labour peaked during the industrial revolution, so there is no way to know how they came to that conclusion, but a reasonable explanation is that they counted children working in factories as child labour but they did not count the close to 100% of children working on their parents' farms as child labour, which was the situation before the industrial revolution.