Children with early-onset conduct problems (EOCP) and callous–unemotional (CU) traits are highly resistant to treatment and cost society significantly more than their healthy counterparts. Employing a multiple baseline design, 13 males (9.0–10.2 years of age) with a history of school suspensions were sequentially introduced to KooLKIDS, a school-based interactive multimedia intervention for young children with EOCP and CU. Teacher and child self-report data revealed significant reductions for the group in proactive and reactive aggression, and antisocial traits. Significant changes in the desired direction were also found on measures of emotion regulation. When the data were analyzed for each individual child, however, the magnitude of any effects for behaviour change varied greatly. KooLKIDS offers potential for treating children with EOCP and CU and hence preventing progression towards more serious antisocial behaviour.