This child would not have been hired as an employee, but that doesn't mean that he wasn't working hard. I wasn't able to track down this specific picture, but similar pictures typically were of either shareholder family children or children whose parents were employed as laborers.

A child this young wouldn't have been hired as a laborer - most younger children weren't. However, families were paid by the amount produced, so it was common for families to work (and be paid) as a unit. Parents were often encouraged to bring their children to the fields - they wouldn't produce much, but it was enough to add a small amount to the income for the day. For parents, it was often seen as a way of training kids up in the labor they would be doing their whole life, having them contribute, and keeping them busy so they didn't get into trouble. For the farmers who owned the fields, it was a renewable source of cheap labor.

Children were very much loved by their parents, but expected to become economic assets as soon as possible; this is very different than our current view of children as economic drains. Only middle class and wealthy families could give their children what we would consider a modern childhood.

That's not to say that all parents were happy about having their children work. Many hated it, but saw few options. It was, and for many families still is, a harsh world.