Studies that measure the effectiveness of language-training program for children's language development do not clearly identify in adult-child interaction which characteristics of educators' language are most effective, particularly for children over 4 years of age who need to develop complex syntactic skills. Our study aimed to analyze the language use of 26 educators (with different professions: teachers, animators, and employees) participating in French language-training program for preschoolers. This training was based on linguistic research on language acquisition and focused on syntax. A total of 131 children were recorded in adult-child joint narration in a dyadic interaction across six French cities and 20 preschools. The statistical analysis of the linguistic and pragmatic properties of the educators' language while interacting with children demonstrated that the educators changed their language practices after training. This study also revealed that syntax complexity of educators' input has a significant impact on syntax development of children.