This study investigated developmental and sex-related differences in affective decision making, using a two-deck version of Children's Gambling Task administered to 3- and 4-year-old children. The main findings were that 4-year-old children displayed better decision-making performance than 3-year-olds. This effect was independent of developmental changes in inductive reasoning, language, and working memory. There were also sex differences in decision-making performance, which were apparent only in 3-year-old children and favored girls. Moreover, age predicted awareness of task and the correlation between the latter and decision-making performance was significant, but only in 4-year-old children. This study thus indicates that there is a remarkable developmental leap in affective decision making, whose effects are apparent around the age of 4, which according to our results, also marks the age when the correlation of declarative knowledge and decision-making performance becomes significant.