Infant and young child feeding (IYCF) interventions in low-resource countries mainly target pregnant women and mothers of young children; however, fathers and grandmothers also influence IYCF practices. We conducted focus group discussions with mothers, fathers and grandmothers of young children across three time points in areas where an IYCF social and behaviour change intervention was implemented in Nigeria to explore differences by participant type and shifts over time in attitudes, beliefs and social norms related to breastfeeding and dietary diversity (DD). Overall, across time points, we found more discrepancies in attitudes, beliefs and social norms for early initiation of breastfeeding (EIBF) and exclusive breastfeeding (EBF) among different participant types than for DD. Although most participants agreed EIBF and EBF are good practices, mothers believed this more strongly than fathers and grandmothers; however, at endline, a shift towards acceptance of EIBF and EBF appeared among fathers and grandmothers. Across time points, all participant types acknowledged the nutritional and health benefits of green leafy vegetables and animal-source foods but described various barriers to feeding them to children. Across time points, all participant types also highlighted the importance of health workers and antenatal visits as important sources of IYCF knowledge and facilitators to following recommended practices. Insights from this study highlight the importance of including key influencers of IYCF practices in qualitative research.