Children with complex disabilities such as autism spectrum disorders and Landau Kleffner syndrome often lack means to participate in everyday family routines. Serious problem behaviors may result from their challenges in responding to and initiating communicative interactions. These behaviors can change routine family activities such that the child and other family members (parents, siblings) are dissatisfied with these routines. The purpose of this study was to examine the use of functional assessment and positive behavior support carried out in equal partnership with family members to reduce a child's challenging behavior and increase his or her engagement in three family-chosen home activities. A multiple-baseline design across routines was used to determine the effectiveness of intervention in reducing challenging behavior and increasing engagement in the routines. Additionally, the study explored outcomes for positive and negative parent-child interactions within the three -targeted routines. Following parent implementation of positive behavior support, results indicated (1) reductions in challenging behavior, (2) increases in the child's engagement, (3) increases in positive parent-child interactions, (4) decreases in negative parent-child interactions, and (5) increased number of days that the child slept through the night. Social validation by parent observers provided additional support for the effect of the intervention on the child's behavior and child-parent interaction. (Contains 4 tables and 3 figures.)