Social inequality is a persistent global issue which many countries, governments, and policymakers aim to address. The development of language and communication skills during the early years of a child’s life are vital for school readiness, educational success, and later life outcomes. As part of a collaborative research project we sought to bring together data from cohort studies in three European countries; Germany, the Netherlands and the UK. Firstly, with the aim of applying a social classification system (CASMIN) to harmonise the measure of maternal education across the different countries, and then exploring child language skills outcomes prior to entering formal education based on maternal education. Maternal education was successfully harmonised using CASMIN, but the measures of child language within the studies were not directly comparable. Descriptive analysis of maternal education and child language skills by each country was conducted and showed a consistent significant difference between children’s language skill scores based on their mother’s level of education (children with the lowest scores had mothers with a low education level). These findings add to the existing body of evidence demonstrating the persistence of inequalities in early childhood and highlight the continued need for dedicated initiatives in the early years of a child’s life. More broadly we successfully applied CASMIN to education data in all three cohorts demonstrating for future cross-cohort research the potential of CASMIN to harmonise measures of education.