The objective of this study was to examine the contribution of distal and proximal family factors to mental health of Russian children before and during the COVID-19 pandemic. Participants were primary caregivers of 1,985 children aged 6 to 12 years. The 2011-2019 sample consisted of 1403 children (53% girls; mean age 8.6; SD = 1.2); 2020-2021 sample consisted of 582 children (50% girls; mean age 8.8; SD = 1.4). The study used internationally recognized measures with established validity and reliability. Children’s mental health was measured by the Strengths and Difficulties Questionnaire (SDQ); family factors were measured by the socio-demographic questionnaire, short Family Cohesion Scale, and the Alabama Parenting Questionnaire — Brief Form (APQ-BF). The results showed that during the COVID-19 pandemic, parents of primary schoolchildren rated family cohesion lower than before the pandemic; they were also less likely to apply positive parenting practices, and more likely to use inconsistent discipline. High level of parental involvement maintained an existing level of children’s prosocial behavior during the COVID-19 pandemic, whereas with the average and low levels of parental involvement the expression of children’s prosocial behavior during the pandemic decreased. An increase in children’s internalizing and externalizing problems during the pandemic was observed only in families low in cohesion. Low level of parental education weakened the negative impact of child mental health problems on their everyday life, whereas high level of parental education was a risk factor of greater impact. These results can be used to develop evidence-based programs for prevention and intervention of mental health problems in primary school children and their families.