Ending child labour, having a minimum wage, and having the barest minimum of employment safety standards also has a negative impact on economic activity, and bankrupts some employers. [1]
Hell, If our goal was maximizing economic activity at all costs, we'd be rounding up idlers, and putting them to work, involuntarily.
[1] Ones whose business model relies on one of the following:
* Exploitation of children.
* Providing labour-intensive products for less then the cost of keeping a labourer alive, housed, and fed.
* Running machine shops where workers have to juggle chainsaws, while breathing in asbestos dust, and licking radium-coated postage stamps.