Do not try this on a tourist visa, immigration will send you back if they find out.
This article (in German) describes a similar case: a 20 year old girl wanted to visit her relatives(!) in the US for a few months between school and university to improve her English skills. Prior to the trip, on facebook, she had told her relatives that during her stay, she would be able to fetch their kid and the kid of the neighbors from school and babysit them, on occasion. The immigration officer wondered why anyone would want to stay in Cleveland/Ohio for several months as a tourist, started questioning the girl, read her facebook journal, and decided to send her back because babysitting qualified as incompatible with the tourist visa. The article states that the facebook posts did not mention any monetary compensation.
Although the article is a bit sensational (poor little girlie treated rough by unfriendly and stubborn immigration officers), the main facts seem to be true, and if babysitting for your relatives and their neighbors qualifies as work, then working as a farmhand surely does.