Despite the compelling nature of goodness of fit, empirical support has lagged for this construct. The present study examined an interactional approach to measuring goodness of fit and prospectively explored associations with mother–child relationship quality, child behaviour problems and parenting stress across the preschool period. In addition, as goodness of fit might be particularly important for children at developmental risk, the presence of early developmental delay was considered as a moderator of goodness‐of‐fit processes. Children with (n = 110) and without (n = 137) developmental delays and their mothers were coded while interacting in the lab at child age 36 months and during naturalistic home observations at child ages 36 and 48 months. Mothers also completed questionnaires at child age 60 months. Results highlight the effects of child developmental risk as a moderator of mother–child goodness‐of‐fit processes across the preschool period. There was also evidence that the goodness of fit between maternal scaffolding and child activity level at 36 months influenced both mother and child functioning at 60 months. Findings call for more precise models and expanded developmental perspectives to fully capture the transactional and dynamic nature of goodness of fit. Copyright © 2016 John Wiley & Sons, Ltd.