It could be that they bet on a school - related matter. The son bet that he would get an A on a difficult exam, and the mom was skeptical. When he did get the A, the mom lost the bet. So, she had to cook his favorite meal for a whole week.
Perhaps they bet on a sports event. The son was really into a particular team and the mom thought otherwise. When the son's team won, the mom lost the bet. As a result, she had to let the son choose the movie for their family movie night.