The aim of this research was to study how children's play behaviour was related to their cognitive skills and vocabulary development in integrated early childhood special education (ECSE) groups. The longitudinal study is part of the LASSO research project, which concerns children's stress regulation, learning and quality of early childhood education, including special education. Children with and without special needs should be supported according to their diversity of individual needs as equal members of a learning community. The study aimed to discover how this was realised by assessing children's various learning paths. The data were collected between 2012 and 2015 and involved 289 children, including 121 with special educational needs (SEN). The children's cognitive skills and vocabulary were measured with standardised psychological tools: the Wechsler Preschool and Primary Scale of Intelligence (WPPSI) and the Developmental Neuropsychological Assessment (NEPSY). Play behaviour was evaluated with the Preschool Play Behaviour Scale (PPBS). In this study, we hypothesised that progress in children's social play boosts cognitive functions and duly boosts learning. The results indicated that all skills improved for all of the children during the research period, although there were differences in results between children's status groups.