Abstract10% of children worldwide live in households without a biological parent, and 5.4 million children live in residential care institutions. This study describes a participatory, child-informed process of developing a multidimensional measure of child subjective well-being tailored towards the priorities of children who have lived in residential care. Eight focus groups were held with n = 49 adolescents reunified with family after living in residential care in Kenya and Guatemala and six focus groups were held with n = 29 young adults who had lived in residential care during childhood. After analysis of the focus groups, and using the Orphans and Vulnerable Children Wellbeing Tool as a foundation, the resulting tool contained 43 survey questions. Member checking, translation, and cognitive interviewing were conducted. The survey was administered to N = 180 young people in Kenya and Guatemala who were reunified with family after living in residential care or at risk of entering residential care. Exploratory factor analysis indicated that the measure had three salient factors with good convergent validity and internal consistency: care and safety (12 items), basic needs (13 items), and leisure and freedom (7 items). This study contributes a new, psychometrically validated survey measure that can be used to assess the well-being of children connected to residential care, as well as a replicable model for creating contextualized quantitative measures via child participation that can inform policymaking on children’s care in low- and middle-income countries.