Supporting children to successfully transition from primary to secondary school is of utmost importance for several reasons, including to prevent future emotional and behavioural problems. Level Up is a novel, UK-based intervention consisting of five online group sessions, straddling the summer holidays, and providing at-risk children and their parents/carers with skills to manage their behaviour, emotions, and relationships to support their transition to secondary school. A prior evaluation of Level Up reported a need to better describe the mechanisms of change. This study therefore evaluated the experiences of children and their parents/carers regarding the facilitators and barriers to engagement and change, and the perceived impact. Fourteen children and 17 parents/carers were interviewed. Identified barriers and facilitators were: (1) Having a safe, supportive, and fun space, (2) Learning through connection, (3) A family approach, (4) Problematic group dynamics, and (5) Connecting through video calls. Perceived impact was described as: (1) Empowering children, (2) Supporting children socially, (3) Supporting parents and carers in their parenting role, and (4) Supporting a successful transition to secondary school. Another theme (5) describes some families' experiences of limited impact. These findings can be used to better understand how to support children in their school transition.