In various countries preventive support groups are offered to children of mentally ill and/or addicted parents to reduce the risk that they will develop problems themselves. This study assessed the effectiveness of Dutch support groups for children aged 8–12 years old in terms of reducing negative cognitions; improving social support, competence, and parent–child interaction (direct intervention goals); and reducing emotional and behavioural problems (ultimate intervention aim). Children from 254 families were randomly assigned to the intervention or a control condition. Parents and children completed questionnaires at baseline and 3 and 6 months later. Emotional and behavioural problems of intervention group children were also assessed 1 year after the start. Univariate analyses of variance showed that children in the intervention group experienced a greater decrease in negative cognitions and sought more social support, immediately after participation and 3 months later, as compared to control group children. They also remained stable in their feelings of social acceptance (competence aspect) immediately after the intervention, whereas these feelings declined in control group children. The intervention and control groups both improved over time in terms of cognitions, competence, parent–child interaction and emotional and behavioural problem scores. Additional improvement in terms of problem scores was found in the intervention group 1 year after baseline. Further enhancement of effectiveness requires re-consideration of the support group goals; it should be studied whether the goals reflect the most important and influential risk and protective factors for this specific population. Besides, effects should be studied over a longer period.