This study examined the mediating roles of children's callousness and negative internal representations of family relationships in associations between family instability and children's adjustment to school in early childhood. Participants in this multimethod (i.e., survey, observations), multiinformant (i.e., parent, teacher, observer), longitudinal study included 243 preschool children (Mage = 4.60 years) and their families. Findings from the lagged, autoregressive tests of the mediational paths indicated that both children's callousness and negative internal representations of family relationships mediated longitudinal associations between family instability and children's school adjustment problems over a 2‐year period (i.e., the transition from preschool to first grade). Findings are discussed in relation to the attenuation hypothesis (E. J. Susman, 2006) and emotional security theory (EST; P. T. Davies, M. A. Winter, & D. Cicchetti, 2006).