What happens if a child is breastfed for more than two years, and is it considered unlawful if he is breastfed for a few extra days or a month, and what is the expiation for that?

Nursing a child for more than two years is usually not beneficial to the child because Allah made the completion of nursing two years, and after that, the child is not satisfied with milk and needs other foods. However, legally, there is no prohibition against nursing after two years.