Kindergarten children enjoy playing games, as games bring motivation and active learning. Many board and card games require mathematical competencies and, therefore, carefully selected board and card games could be used as meaningful learning tasks for mathematics education in early childhood. With this in mind, several games for fostering quantity-number competencies have been implemented in an intervention study. The study included 6 years old children and three conditions: training program (n=110), play-based intervention (n=89) and control group (n=125). For this article, videos involving 21 children in ten teams from one of the most widely played games in the play-based intervention were selected for in-depth analysis: the board game Shut the Box. This explorative analysis of children’s mathematical behaviour and peer support whilst playing a board game allows researching in what way a specific game provides a meaningful learning task for early mathematics. The results show that children employ several mathematical skills while playing, depending on their individual quantity-number competencies. As the game can be easily repeated several times, the children practise and shape mathematical skills and monitor and support their co-player in order to increase their chances of winning the game. It is suggested that the board game provides an adaptive and motivating setting, which can meet the learning needs of low as well as high achieving children.