I'm pretty sure this would be classified as child labor, but it isn't forced. He wanted to try mowing the lawn and we weren't going to turn down a willing mower! He mowed most of the front lawn, did a great job, and was very excited about getting paid for it.
He had a hard time not leaving strips and tufts of long grass in between rows, but got much better as he went along. We have worked out a deal...$3 for the front lawn with tufts, $4 for the front lawn with just a few tufts, and $5 for the front lawn with no tufts.