This paper addresses the history of voseo, the etymologically plural second person used as singular (vos tenés vs. tú tienes 'you have'), in Río de la Plata Spanish. It shows that the present verbal paradigm resulted from replacement of tuteo by voseo in a specific order (imperative > present indicative > present subjunctive), a sequence which parallels the order of acquisition of these forms and their level of clausal embeddedness. Theoretically, it is argued that the sequence of innovation is evidence of the impact of child acquisition on language change. Voseo started making inroads over tuteo in forms that emerge earlier in child language and occupy the highly salient left periphery, whereas forms in the main clause, which appear later, took longer to shift. In the most embedded positions where acquisition is completed last, voseo adoption was slower and eventually receded.