Children younger than three years old are able to detect hidden rules in numerical sequences, and this ability matches that of adults by age seven. However, the developmental trajectory of this ability during the ages of four to six remains unknown. The present study adopted a modified Brixton task to address this issue. In this task, children were presented with sequences of moving circles and were asked to predict which circle would next turn blue based on hidden rules that were either simple (e.g. + 2) or complex (e.g. + 2 - 1). Results suggested that (a) four-year-olds were only able to detect comparably few simple rules, whereas children older than 4.5 years were able to successfully detect most of the simple rules hidden in number sequences; (b) although all children performed significantly poorer when attempting to identify complex rules as compared with simple rules, rule detection (RD) ability improved rapidly with age, and children older than five were able to identify most complex rules. These findings extended previous work on rule learning by revealing the developmental trajectory of RD among preschoolers.