For most of human history, though, the average person's labor was either relatively relaxed work that wouldn't be ruined by a childish personality (i.e. hunting for nuts and berries in prehistoric humans) or it was back-breaking labor for a very short portion of the year as in pre-industrial farm families. They worked to extremes that would make slaves blush during harvest and planting seasons, but spent the rest of the year essentially free. There was minor work to be done in order to sustain a household, but it was nothing a child shouldn't be expected to handle. I'm sure there were exceptions (probably miners, if I had to guess), but they weren't widespread social issues.
Basically, we only invented child labor laws because we invented forms of labor that a child's body is capable of handling, but their mind isn't. The idea of "children as smaller versions of their adult selves" is very much a post-industrial thing as far as I'm aware.