Maternal depression is a risk factor for future mental health problems in offspring, with stress-system function as a candidate vulnerability factor. Here we present initial validation of an online matching-task paradigm in young children exposed to maternal depression (N=40), a first in stressor-paradigm research for this age group. Investigations of stress-system reactivity that can be conducted online are an innovative assessment approach, accelerated by the COVID-19 pandemic. Results indicate high feasibility, with ~80% success across measures, similar-to or better-than in-person success rates in young children. Overall, the online matching task elicited significant HR but not cortisol reactivity. Individual differences in child mental health symptoms were a moderator of reactivity to the stressor such that children with lower, but not higher, behavioural problems exhibited the expected pattern of cortisol reactivity to the online matching task. Results are aligned with allostatic load models, which suggest down-regulation of stress-system reactivity as a result of experiencing adversity and mental health vulnerability. Consistent with in-person research, this suggests an early phenotype for the emergence of behaviour problems may be linked to altered stress-system reactivity. Results hold potential clinical implications for intervention development and the future of online stress-system research.