Whether children in migrant households maintain proficiency in their heritage language (HL) may affect their attachment to their cultural heritage, while the extent to which they acquire proficiency in the new language of their destination strongly conditions their success in the country's educational and occupational institutions. In this paper, we investigate the extent to which children of Moroccan heritage living in Antwerp, Belgium maintain their proficiency in their parents' HL and the extent to which they learn and speak Dutch. Our research design draws from family language policy to consider how elements of the children's family background, language practices by the parents and siblings, attitudes towards the HL and Dutch, and language management, affect the children's levels of proficiency in each language. Based on data for over 300 children, the results show that the children's proficiency in their HL is strongly affected by whether the parents use and value the HL, the mother is proficient in the HL, and by opportunities to use the HL outside of the household. In contrast, the children's proficiency in Dutch is affected by the mother's proficiency in Dutch and by the languages used by the other children in the household.