This paper addresses diglossic variation and language acquisition in children growing up in diglossic speech communities, whereby two interrelated language varieties are associated with different levels of exposure and use at different stages of development. We present evidence for the interplay between Arabic language varieties in diglossic acquisition of phonological, morphosyntactic, lexico-semantic and pragmatic knowledge; and discuss the need to reformulate the examination of linguistic developmental trajectories to reveal interactions between language acquisition and the sociolinguistic characteristics of diglossia. We highlight the importance of this work for enhancing the applicability of clinical and educational services for diglossic speech communities.