Fetal exposure to prenatal maternal stress can have lifelong consequences, with different types of maternal stress associated with different areas of child development. Fewer studies have focused on motor skills, even though they are strongly predictive of later development across a range of domains. Research on mechanisms of transmission has identified biological cascades of stress reactions, yet links between psychological stress reactions are rarely studied. This study investigates the relationship between different aspects of disaster-related prenatal maternal stress and child cognitive and motor development, and proposes a cascade of stress reactions as a potential mechanism of transmission. Mothers in the Queensland Flood Study (QF2011) exposed to a major flood during pregnancy completed questionnaires assessing flood exposure, symptoms of peritraumatic distress, dissociation, and posttraumatic stress (PTSD), and cognitive appraisal of the overall flood consequences. At 16 months post-partum, children's (N = 145) cognitive and motor development was assessed using the Bayley-III. Flood exposure predicted child cognitive development and maternal PTSD symptoms and negative cognitive appraisal were significantly negatively related to child motor development, with all relationships moderated by timing of exposure. Together, a cascade of stress reactions linked maternal flood exposure to poorer fine motor development. These findings suggest that the way stress reactions operate together is as important as the way they operate in isolation, and identifies a potential psychological mechanism of transmission for the effects of prenatal stress. Results have implications for conceptualizing prenatal stress research and optimizing child development in the wake of natural disasters.