In this study we adapted the Computerized Comprehension Task (CCT), to be used in rural native communities with bilingual children learning Qom Indigenous language and the Chaco dialect of Argentinean Spanish. The main objectives were (a) to test the utility of this task, which provides accuracy and response time measures of decontextualized vocabulary, for use with children from three to seven years in schools and rural settings; (b) to assess Qom children’s vocabulary learning trajectories in both languages. The adaptation of the CCT was based on extensive fieldwork establishing language usage patterns, word frequency, and age of acquisition. Eighty-three children from three to seven years of age living in Chaco Province were assessed in Qom and Spanish, employing the CCT adaptation. Findings provided evidence of attrition of children’s heritage language (which is on par with vocabulary in the majority language up until age five), beginning as children approach elementary school.