Primary caregiving grandparents can play an important role in young children's development including the development of healthy lifestyle behaviours. This study aimed to gain insight into the perspectives of primary caregiving grandparents regarding the promotion of healthy eating and activity behaviours in their 1–7-year-old grandchildren. Semi-structured interviews (45–90 min) were conducted with primary caregiving grandmothers (n = 7). Interviews investigated grandmothers' beliefs, knowledge, perception of their capacity, confidence, ability, and the barriers and facilitators to promoting healthy lifestyle behaviours in their grandchildren. Thematic analysis revealed four themes: (1) Influence of past experiences on grandparenting; (2) Grandparenting role and responsibility; (3) Intrinsic drivers of grandparenting; and (4) Confidence and capacity to grandparent. Grandmothers adopted a different and improved parenting approach with their grandchildren based on their life experiences and high sense of responsibility to improve their grandchildren's wellbeing. They expressed a high perceived level of knowledge and confidence in their ability to promote healthy lifestyle behaviours in their grandchildren, and demonstrated positive attitudes and practices towards healthy eating and physical activity. Additional support was inconsistently desired. Further research is necessary to identify priorities and suitable intervention strategies supported by policy development to enhance the health and wellbeing of grandparent carers and their grandchildren.