One in six school-aged children now have a probable mental health disorder -- up from one in nine three years ago (NHS digital, 2020). Services for children and young people’s mental health are stretched more than ever before, such that maximising resources is a key priority (Lennon, 2021). The effectiveness of mental-health interventions can be optimised by enhancing our understanding of the mechanisms of change – that is, how an intervention translates into events that lead to improved outcomes (Kazdin, 2007). Parent and child behaviours are primary intervention targets for children’s mental health (WHO, 2009), yet little is known about their assumed mechanistic role, that is, how parent-child dynamics change as a function of, or during intervention. We aim to address this gap, with specific focus on children’s disruptive behaviour as one of the most common reasons for children to receive mental-health services (Achenbach & Edelbrock, 1987; Hinshaw & Lee, 2003).