Child outcomes vary by family’s socioeconomic status (SES). Research on explanatory factors underlying early SES-related disparities has mainly focused on specific child outcomes (e.g., language skills) and selected influencing factors in single countries often with a focus on individual differences but not explicitly on early SES-related gaps. This study uses harmonised data from longitudinal large-scale studies conducted in the United Kingdom, United States, and Germany to examine parental education-related gaps in early child language and social skills. Twelve theoretically proposed family-, child-, and childcare-related factors were systematically evaluated as explanatory factors. In all countries, parental education-related gaps were particularly pronounced for early child language compared to social skills. In the decomposition analyses, the home learning environment was the only measure that significantly explained gaps in all child outcomes across all countries. Early centre-based care attendance, family income, and maternal age at childbirth contributed to gaps in child outcomes with the specific pattern of results varying across outcomes and countries. Maternal depressive feelings significantly contributed only to explaining gaps in children’s social skills. Thus, while some mechanisms found to underpin early parental education-related gaps can be generalized from single-country, single-domain studies, others are outcome- and context-specific.