This article makes a case for attending to the specificities of child illegality in migrant contexts. This is not simply because children have been left out of previous accounts, but also because their status as minors makes both their citizenship and their illegality different to that of adults. The analysis is based on research with children born to migrants in the state of Sabah, East Malaysia. I argue that such children are configured as Sabah’s impossible children, and that this configuration influences their experiences of illegality and exclusion in distinctive ways. From a young age, children are aware of document ‘checking’ raids and, as ‘foreigners’, are unable to attend Malaysian schools. However, informal documents from learning centres, as well as age and contingent circumstances, may give them a temporary, ‘liminal’ legality. Finally, given that irregular migrants experience both exclusion and inclusion in a host nation, the article describes children’s urban forms of belonging. These forms of inclusion demonstrate children’s engagement with Sabah as a home, as against their political construction as an impossible problem.