Young learners' use of instructional games in foreign language learning is not yet well understood. Using games that were part of the learning tools for an online assessment, Jido-Eiken, a standardized English proficiency test for young learners in Japan, we examined young learners' game-playing behaviours and the relationship of these behaviours with learning outcomes. The participants were 3,945 children aged 4 to 12. We found that games that children played with relative frequency tended to share a set of features, including being cognitively demanding, evoking one's curiosity, offering greater player control, and having multiple players. The frequency of plays decreased as the age increased in many games that we examined, but gender differences were hardly observed. We found different relational patterns between young learners' game scores and English performance depending on the games and difficulty levels of the assessment. We discuss the implications for instructional game developers and teachers of young language learners.