I believe the parent comment's point would be better formulated as: We make an exception for child actors but do so thoughtlessly; if we applied the same lines of moral reasoning that leads us to prevent children from working elsewhere, many of those lines would prevent us from making an allowance for paid child actors. But we fail to make the application of those lines of reasoning in this case because of cultural loopholes and blinders, not any limitation in the reasons themselves.

The one line of reasoning I can think of that does limit itself in this regard is the one says "there are no good adult substitutes for child actors, hence an exception," but this is a very cynical moral ground for child labor laws, as there are probably many potential exceptions that could be forced here that we would find profoundly repugnant and we would have to abandon that line of reasoning or be forced to admit we are more interested in industry outcomes than the welfare of children or our moral worth.