Don't allow the child to sense that you are (in any sense) disappointed in him or her. That advice is for parents, teachers, or anyone who has ongoing contact with that child. Not showing disappointment doesn't mean you put up a front with the child. Instead, a parent or teacher should engage in some honest soul-searching. This kind of introspection requires that we, parent or teacher, are completely honest with ourselves. First, children are not underachievers because they want to disappoint anyone. In all probability, they need additional help with the area they are struggling with. They do need to know that their parents love them unconditionally. I struggled with math all through elementary, middle, and high school. My parents told me, "Just do your best. If you have tried, and your best is a failing grade, you have done what we asked." This advice caused me to attempt to do things that I thought were extremely difficult, and sometimes I wasn't sure I could accomplish my goals----but my parents removed the fear of failure with those words. I have continued to set goals that appear difficult, and I am grateful that my parents let me know early in life that their love wasn't contingent on bringing home high math grades. (I know I answered this question with a positive answer rather than saying what parents should NOT do....this feels better to me).
Hope that's okay with others who read my answer.