Although many young children become literate within an environment in which different writing systems exist, there is little research on what children know about different writing systems and how they understand and develop them when they are learning more than one simultaneously. This qualitative study discusses how Korean EFL (English as a Foreign Language) children understand two different writing systems, the Korean alphabet, Hangul, and the Roman alphabet, used for English, within a peer teaching setting. The findings show that they were able not only to discover key orthographic principles which characterise each writing system but also to find similarities and differences between Hangul and English from different points of view: shapes of letters (block shaped vs linear), language units (syllables vs letters) and sound-letter relationship (shallow orthography vs deep orthography). The paper suggests that young children are able to look for key concepts in different writing systems by constructing their own ideas about the principles of reading and writing from an early age as active language learners.