The "million-dollar question" is designed to be difficult enough to stump even adults who haven't recently reviewed elementary school material, even though the underlying concepts might be within the grasp of a 5th grader who's paying attention in class. The difficulty comes from the pressure of the situation, the time constraint, and sometimes the way the question is worded or framed.
In short, it's a 5th-grade *level* question, meaning the subject matter is drawn from that grade's curriculum, but not necessarily a typical or easy 5th-grade question. The show uses the clever wording to create a high-stakes challenge.