Success and failure in formal mathematics education has been used to legitimize stratification. We describe participatory design research as a methodology for systemic repair. The analysis describes epistemic authority--exercising the right or the power to know--as a form of agency in processes of mathematical problem solving and learning. We asked: What will aid families in advocating for their children's math learning, particularly when they expressed concern about their ability to do so? Participatory design research provided a collaborative and iterative method to work with people who shape math learning: parents, children, teachers, community organizers, researchers, curriculum developers, and mathematicians. Data from four years of participant observation involved the design, facilitation, and dissemination of workshops and take-home materials and family case studies. As participating families claimed epistemic authority, institutional barriers became more visible. This tension maps where participatory design methodology can evolve to address systemic change.