Vocabulary growth in preschool strongly predicts later reading and academic success, yet multi- and bilingual children with migration backgrounds may have unequal access to sustained exposure to the language of instruction. This exploratory mixed-methods study examined vocabulary development and app engagement among 72 multi- and bilingual preschool children in Switzerland who used a multilingual picture-book app over three months. Receptive-vocabulary pre- and post-tests indicated within-group gains, and the number of books opened was moderately associated with vocabulary-score change. Qualitative interviews with 14 educators, parent surveys, and child feedback highlighted adult scaffolding, parental digital literacy, timing, child interest, and concerns about passive screen time as important implementation conditions. Because the study used a one-group pretest-posttest design without a control group, findings should be interpreted as exploratory associations rather than evidence of causal app effectiveness.