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.
