capitalism came with machines that mangles children, chemical exposures etc. Things they can't possibly know the consequences of.
Realistically like a kid making a basket or watching sheep is not the same danger level as being pulled into a machine or getting silicosis before they turn 18
Because it's a system, not a person. It has no volition to "fix" anything.
People living within the system can and have opposed child labor, as others have pointed out. I know that socialists, once in power, have the ability to snap their fingers and eliminate all the complexities and challenges of running a globalized industrial society, but unfortunately the rest of us have to live in a world where not everything has an easy, cost-free solution.