Studies have recognized the increasing importance of mother’s educational attainment in shaping children’s life chances. This study explores how maternal employment contributes to disparities by mother’s education in child well-being via two processes: (1) educational differences in the composition of maternal employment; and (2) educational differences in relationships between maternal employment and child outcomes. In doing so, this study goes beyond prior research by focusing on different types of employment (nonstandard versus regular) and its long-term pattern. Japan’s distinctive family, education, and labor market contexts provide alternative scenarios regarding how maternal employment and its relationship with child outcomes may vary by mother’s education, and its contribution to unequal child outcomes. Analyses using marginal structural models reveal limited differences in long-term employment between highly and less-educated Japanese mothers, but marked discrepancies in relationships between cumulative exposure to maternal employment and children’s outcomes by mother’s education. Observed detrimental impacts of mother’s work are almost exclusively limited to children with less-educated mothers without a college degree, and more pronounced for regular than for nonstandard jobs. Counterfactual predictions suggest that educational gaps in Japanese children’s outcomes primarily reflect differential relationships rather than compositional differences in maternal employment. These findings offer new insights into families as a nexus of inequality where family processes and child outcomes differ across the socioeconomic spectrum.