Prices reward people who do work that there is an unmet demand for in society. If society already has an oversaturation of professional photographers (so much so that clients feel they do not have to compete at all), the low price of $0 for professional photography work will hopefully encourage some of these glut of professional photographers (and others looking to enter the field) to pursue another profession where they can add more value.
If Netflix's contractor here has set the price too low, well then no one will apply, and they will have to set the price higher.
The problem then is the same as snake oil; the demand exists only because the consumers lack information. And then keeping in mind that market principles only produce efficient economies when information is available and known.
If you put all that together you get a perfect storm of terribly misaligned incentives and vulnerability that make child labor a very different case than what we're discussing here. It's entirely possible or even likely for a rational service provider to throw in a freebie in exchange for intangibles like publicity. It happens all the time in the adult world. So I don't think child labor is a fair thing to compare this situation to.
A prominent example is child labor laws. Before these were passed, the market would bear endangerment and injuries to children because something was better than nothing to families that had very little, and it was rationally in each family’s best interest to take the risk in exchange for some small amount of money. If they had all collaborated in an organized strike, they could have demanded better conditions, but in practice that wasn’t going to happen for people that were already extremely constrained on money and time (and all it takes is a minority of defectors to prevent this from working). It creates a feedback cycle of compounding inequality, and the only way to break it is via regulation.
So once you allow unpaid work in exchange for "exposure", that becomes the new normal, and no individual photographer can alter the system while still acting in their own best financial interest.