Determining the causes of working memory maturation is a global issue in developmental cognitive psychology. The question has most often been addressed by manipulating stimulus types within participants. In the present experiment, we instead addressed this question by examining development in children’s working memory using a single stimulus type. Primary school children aged 6 to 10 years (total N = 256) participated in a task inspired by the game Simon®. Children saw sequences of colored squares presented one-at-a-time, and then attempted to reproduce the sequences in the correct order. Some sequences were simple and contained regularities that made them compressible, while other sequences were more complex and less compressible. We found that children’s memory for both kinds of sequences improved with age. However the ratio between children’s memory for simple and complex sequences did not substantially change with age. These ﬁndings suggest that storage capacity of working memory increases in middle childhood, but that children’s ability to optimize storage by compressing sequences remains stable across this age range.