This paper focuses on vocabulary development in oral language productions of three elementary school-age sibling pairs growing up in a trilingual setting. This longitudinal study describes the development of the children's narrative competence over three years. The corpus analysed consists of retellings of animated films. The contribution deals with the lexical development in the three languages of the children, in particular the length of the productions, lexical diversity and lexical sophistication. Lexical diversity is measured by the Guiraud Index. It is assumed that the children's vocabulary becomes more diverse with increasing age. Lexical sophistication is examined by measuring the average frequency of words used by the participants. It is assumed that as children's vocabularies grow, they will include more and more 'rare' low-frequency words in their productions. Assuming that the school language is the dominant language, the question is whether lexical diversity and sophistication increase with age in all three of the children's languages, or whether this is only the case in the school language. The results show that lexical development varies from child to child. However, a certain trend can be observed, indicating that school vocabulary is becoming more sophisticated.