Background: Interparental violence is both common and harmful and impacts children's lives directly and indirectly. Direct effects refer to affective, behavioral, and cognitive responses to interparental violence and psychosocial adjustment. Indirect effects refer to deteriorated parental availability and parent-child interaction. Standard Trauma Focused Cognitive Behavioral Therapy may be insufficient for children traumatized by exposure to interparental violence, given the pervasive impact of interparental violence on the family system. HORIZON is a trauma focused cognitive behavioral therapy based group program with the added component of a preparatory parenting program aimed at improving parental availability; and the added component of parent-child sessions to improve parent-child interaction. Methods/design: This is a multicenter, multi-informant and multi-method randomized clinical trial study with a 2 by 2 factorial experimental design. Participants (N = 100) are children (4-12 years), and their parents, who have been exposed to interparental violence. The main aim of the study is to test the effects of two parental components as an addition to a trauma focused cognitive behavioral based group therapy for reducing children's symptoms. Primary outcome measures are posttraumatic stress symptoms, and internalizing and externalizing problems in children. The secondary aim of the study is to test the effect of the two added components on adjustment problems in children and to test whether enhanced effects can be explained by changes in children's responses towards experienced violence, in parental availability, and in quality of parent-child interaction. To address this secondary aim, the main parameters are observational and questionnaire measures of parental availability, parent-child relationship variables, children's adjustment problems and children's responses to interparental violence. Data are collected three times: before and after the program and six months later. Both intention-to-treat and completer analyses will be done. Discussion: The current study will enhance our understanding of the efficacy interparental violence-related parental components added to trauma focused cognitive behavioral group program for children who have been exposed to IPV. It will illuminate mechanisms underlying change by considering multiple dimensions of child responses, parenting variables and identify selection criteria for participation in treatment.