The development of domain-specific competencies and the emergence of social disparities start well before school entry. These early developments have been suggested to be highly relevant to later developments, educational pathways, and participation in society. Longitudinal large-scale studies, in particular, provide important insights into relevant individual preconditions, developmental trajectories, and their relation to learning opportunities in different learning environments. Against this background, this paper presents selected results of the longitudinal and interdisciplinary study BiKS-3-18 with a special focus on education-related facets of child development at preschool age, their interrelations, predictive impact, and connection to environmental conditions. In particular, we (1) present results on early emerging individual differences between children, their stability over time, and their relation to children’s socioeconomic family background (SES). (2) With a special focus on language development, we address the impact of child characteristics and the dynamics of early child development by presenting findings (a) on changing developmental relations between working memory and language acquisition and (b) on the interrelations between early child language and children’s social-cognitive, metacognitive, and social-emotional development. (3) Finally, we report findings on the importance of individual differences and SES-related disparities, particularly in the language domain, for later school-related language competencies and school performance.