Research is scarce concerning the validity of subjective wellbeing measures for children aged 12 years and less. There are even fewer subjective wellbeing data for children with complex backgrounds and personal circumstances and those experiencing socio-demographic disadvantage. This study compares the psychometric properties of the child and adult versions of the Personal Wellbeing Index. Participants were 1761 ‘at-risk’ children aged 10–12 years, and 2000 geographically representative Australian adults. While there was sufficient fit with a one-factor structure, the reliability estimates were lower in the child sample and the incidence of response bias was significantly higher. Collectively, these findings suggest caution in the use of subjective wellbeing measures with children.