This study aimed to investigate the intricate relationship between socio-economic status (SES) and language development in Korean children under 3 years old. Specifically, it sought to understand how SES influences language development and explored the interconnections among SES, parent-child interactions (specifically through play and book reading), maternal employment, and children's vocabulary scores within the context of Korean parenting culture. Data were collected from 348 mothers of children aged 8–36 months (168 girls; mean age = 20 months 5 days, SD = 7 months and 7 days). Language development was assessed using the Korean version of the MacArthur-Bates Communication Development Index (K-CDI), and parental play frequency, duration, and book-reading frequency were obtained through a questionnaire. We used structural equation modeling to analyze the impact of SES on vocabulary outcomes, examining mediating connections with parental play and book-reading frequency, as well as children’s vocabulary skills. Additionally, we compared variables (SES, play and book-related factors, and K-CDI scores) between working mothers and stay-at-home mothers. The findings indicated that SES significantly predicted vocabulary size, but this influence was fully mediated by parental play frequency, play duration, and book-reading frequency. Interestingly, maternal work status showed no significant impact on children's vocabulary skills, and there were no differences in play frequency, play duration, or book reading frequency despite higher SES among working mothers. The mitigated role of maternal work status on children’s vocabulary skills, despite the differences in their SES, underscores the importance of parent-child interactions, particularly shared book reading, in vocabulary development when these interactions were considered.