Child language acquisition research has provided ample evidence of lexical frequency effects. This corpus-based analysis introduces a novel frequency measure shown to significantly constrain adult language variation, but heretofore unexplored in child language acquisition research. Among adults, frequent occurrence of a form in a particular discourse context that conditions usage accumulates in memory over time and shapes the lexical representation of that form. This study contributes to the body of research on frequency effects in child language acquisition by testing whether such cumulative conditioning effects are also found among children, and, if so, at what age such effects appear. Specifically, the study investigates the influence of a distributional frequency measure (each verb form's likelihood of use in a switch vs same-reference discourse context) on variable subject personal pronoun (SPP) expression (N = 2227) in Spanish (e.g. "yo voy ~ voy," both meaning 'I go') in the speech of 65 monolingual children in two age cohorts. Results reveal sensitivity to the contextual conditioning of discourse continuity (switch reference) among both the younger (6- and 7-year-olds) and older (8- and 9-year-olds) children in support of previous research. In addition, each verb's likelihood of use in a switch-reference context significantly predicted the SPP use among the older children, but not the younger ones, suggesting that the cumulative effect of a probabilistic pattern takes time to emerge during childhood. The lexically specific accumulation in memory of contextual conditioning effects supports exemplar models of child language acquisition: each instance of use in discourse contributes to the lexical representation of that form and, over time, plays a role in the creation of morphosyntactic patterns during language development.