Purpose: Children with Down syndrome often have poor speech intelligibility, which can mask expressive language competence; this, in turn, can lead to serious misconceptions about overall competence and intellectual abilities. Although aided augmentative and alternative communication (AAC) can be used to bridge these gaps, children with Down syndrome are not always provided with consistent access to focused AAC language intervention supports. The primary goal of this study was to evaluate the effect of implementing the AAC Generative Language Intervention (AAC-GLI) approach on the aided expressive grammar productions of young children with Down syndrome. Method: A randomized controlled trial was used to evaluate the results. The study included a control group and an intervention group, with the families of both groups participating in half-day AAC implementation workshops and all children receiving AAC devices to use throughout the course of the study. The intervention group also received 4 months of twice-weekly play-based AAC-GLI intervention sessions. Progress was measured using a mean length of utterance (MLU) specially designed for aided communicators (weighted MLU in symbols [W-MLUSym]). Results: Strong effects indicated superior performance on W-MLUSym for the intervention group, despite reduced enrollment and increased attrition yielded by the COVID-19 pandemic. Conclusions: AAC-GLI can be used to teach young children with Down syndrome to improve their aided expressive grammar skills. Providing AAC language intervention for young children with Down syndrome can be a critical step to support ongoing expressive language development and use as well as overall functional communication.