This article is based on a qualitative study of the relationships between professionals, parents and children in the context of children's therapy after child sexual abuse. Specifically, the study took place in a therapeutic team in England that has developed various ways of involving parents in children's therapy. One of the aims of the study was to develop an understanding of the complex triangular dynamics that result. In this article, concepts from discourse analysis are sued to show how therapist talk about various types of 'cases'. therapy often follows a child protection enquiry, and so professionals, parents and children must negotiate sensitive issues of blame and responsibility. Second, the article draws on sociological work on expertise to show how therapists employ 'interactional expertise' as they combine their own expert view of the child's situation with the expertise held by the family, whilst being mindful of the imperative of child protection. Only a successful resolution of these tensions can lead to a negotiated agreement over a plan for therapy for the child. It is suggested that this study highlights an example of the kind of interactional expertise that well developed within social work and reflects its distinctive value base.