I don't think it's so unpopular to be suspicious of a blanket law against child labor, but the point about compulsion is critical. Our progressive laws are a compromise that is massively good in the net-that's the nature of the modern world. It may be there's a better arrangement, but it has to avoid the pain, we can't go back to it now. If children can work, poor children will. This recent essay is a good case FOR:
https://letter.palladiummag.com/p/school-is-not-enough
But, it's the kind of thing that cannot be flipped like a switch--it has to, as you say, be part of an effective project of economic justice, which we're very far from.
In lieu of that, the challenge is, can we continue to prevent child labor while compensating for its downsides? Pain is information. Child labor laws trade a widespread and severe pain for a somewhat narrower and vaguer one, but we can respond to that too (a kind of gradient descent).
