Child routines and coparenting relationships are respectively recognized in past literature as valid predictors of child adjustment, representing structured daily living and source of support in children’s lives. Under the difficult times of the COVID-19 pandemic, established family processes were shaken by social and mental challenges, which lacked relevant literature regarding the working mechanism in the Chinese cultural context, where parent-grandparent coparenting families commonly reside. The current longitudinal study explores the role of child routines in the effect of mother-grandmother coparenting relationship quality on child adjustment over the period of the COVID-19 pandemic lockdown. The study collected data from 163 mother-grandmother coparenting families before, during, and after the pandemic lockdown in Beijing, China, in a one-year span. The results showed that the consistency of child routines during the pandemic mediated the effect of mothers’ perceived coparenting relationship quality before the pandemic on child social competence after the lockdown, but not on child behavioral and emotional problems. Grandmother’s perceived coparenting relationship quality before the pandemic negatively predicted behavioral and emotional problems after the lockdown, while no indirect effects were found. The current study reiterates the critical role of routine maintenance in the coparenting context on child social-emotional development, particularly when other parts of daily lives were disturbed. Also, separately examining mothers’ and grandmothers’ perceived coparenting relationship quality validates the idea that caregivers played different roles in the family, resulting in disparities in child development mechanisms.