This study examined the validity and reliability of different measures of morphosyntactic complexity, including the Morphosyntactic Complexity Scale (MSCS), a novel adaptation of the Developmental Sentence Scoring, in French-speaking school-aged children. Seventy-three Quebec children from kindergarten to Grade 3 completed a definition task and a narration task. Mean length of utterance (MLU), clause density and MSCS global score, average frequency scores and average complexity scores were calculated from the transcripts of the two contexts. MLU, clause density and MSCS global score were correlated with vocabulary knowledge and narrative skills, and they increased as a function of school level, suggesting that they are valid measures of morphosyntactic complexity. Moreover, the three scores were correlated across contexts, suggesting that they are also reliable measures. However, no MSCS average frequency or average complexity score was found to be both valid and reliable. These findings will guide researchers and practitioners who desire to assess the language skills of French-speaking school-aged children.