If a math lesson is not fun, then it had better be about math foundations and not something different than the real math. But K-6 math is seldom about math foundations. So if the math lesson is not fun, it does no good to teach the math foundations concept.
The limited willingness of students to pay attention to something not fun should be used for math foundations. If it is not, the student doesn’t learn the concepts as they truly are at the few limited chances there are to give that concept lesson.