Vague programme descriptions are known to impede implementation and evaluation. Yet social work change programmes often fail to describe in detail how they aim to provide better outcomes for children and families. This study focused on the Systemic Practice Model (SPM), which is a Finnish adaptation of Reclaiming Social Work, a practice model developed in England. The SPM aims to deliver systemic practice in children's social care and has recently been widely disseminated across Finland. However, research has found both considerable variation in its delivery and a lack of clarity about what it is. This study, applying realist evaluation, aimed to formulate a programme theory based on the perceptions of key informants (n = 12) involved in the national development and dissemination of the SPM during the period 2017–2019. The analysis yielded three core components (a systemic team, systemic weekly meetings, and systemic practice) and two context-mechanism-outcome (CMO) configurations, which represent causal chains of the SPM. The findings should enable its quality implementation and meaningful outcome and process evaluation. The findings will also aid organizations and practitioners both to identify changes required in their service provision and prevent adverse effects. This programme theory should be tested and refined with empirical data.