This article presents the findings from a critical review of cost-effectiveness research in children’s social care. With a focus on the past 20 years (since 2000), the review aims to assess the use and consistency of definitions relevant to evaluating cost-effectiveness such as financial input (costs) and outcomes, and to review and summarise learnings from cost-effectiveness studies in the English children’s social care system. We included both academic and grey literature and identified 61 relevant publications for inclusion. The results are organised according to three themes: costs, outcomes and cost-effectiveness. We identified that a large proportion of studies contained a discussion on cost, spend or unit cost, and an equal proportion of articles concerned outcomes of service, benefit to children or quality of service provided. The number of studies discussing cost-effectiveness, cost–benefit or economic evaluation was considerably smaller. The findings highlight substantial gaps in the literature, with a disproportionate focus on stating the problem in terms of cost pressures, and very little robust evidence about cost-effectiveness. Furthermore, the article sets out methodological limitations and indicates a lack of transparency in many of the report studies. We conclude that as a result of the gaps and limitations it is difficult for policymakers and other stakeholders in children’s services to make evidence-informed decisions about the best use of their limited resources.