In recent years, mobile English learning app has become a prevalent learning tool for young learners (Truong, 2020). Both teachers and parents should delve into its linguistic principles and pedagogical implications so that they can understand the enablers and barriers of those apps to children's digital literacy (Oakley, 2020). Unfortunately, despite the number of recent studies, the dearth of research has critical scrutiny about the suitability of English learning apps for young learners and the principles underlying the design and functionality of mobile apps for children. As a result, the purpose of this study is to examine Monkey Junior, one of the twenty most downloaded English learning apps for kids in the US (Monkey Junior, n.d.), to provide a breakdown of the principles underpinning its functionalities, which include multisensory, gamification, phonics, and sight words. The research provides a more transparent overview of the academic rationales for the principles reflected in the app's design. The multisensory approach including visual stimuli, auditory techniques, tactile teaching methods, and kinesthetic learning has been maximized in Monkey Junior. Also, both multisensory phonics and sight words have been applied in the app to optimize children's literacy learning. Several types of games have been designed for Monkey Junior to diversify forms of lesson revision and keep children interested in trying a new different game after each unit. Finally, the study will discuss some limitations and what might be missing from this app.