This article examines the production, working and impact of the UK's hostile environment on migrant families with precarious legal status. Our approach is informed by two bodies of scholarly work: critical border studies and research on migrant families. We bridge these literatures to show how the hostile environment is neither a singular, neatly bounded space, nor limited to a set of interactions between immigration enforcement and a clear-cut group of people (so-called ‘illegal immigrants’). It affects the lives of a wider segment of the UK population, in particular racialised migrants and citizens, by making their legal status more insecure and precarious, and percolates in multiple and intersecting domains in the lives of families, such as education, housing and welfare, making them ambivalent sites of protection and safety as well as control and enforcement. Drawing on ethnographic engagement with families with insecure immigration statuses, we explore how the hostile environment manifests in the everyday lives of families; how the hostile environment circulates and is re-enacted within the micro-politics of families; and how families negotiate the continuous work of protecting children from the effects of the hostile environment. In conclusion we argue that dramatic and rapid shifts in immigration rules and regulations undermine the capacity of mothers to navigate the policy environment and welfare for their children and to shield them from the consequences of state-driven hostility towards immigrants.