Joint pretence games are implicit rule-governed activities with a normative structure: Given shared fictional stipulations, some acts are appropriate moves, others are inappropriate (i.e., mistakes). The awareness of 2- and 3-year-old children of this normative structure was explored, as indicated by their ability to not only act according to the rules themselves but to spontaneously protest against 3rd party rule violations. After the child and a 2nd person had set up a pretence scenario, a 3rd character (a puppet controlled by another experimenter) joined the game and performed acts either appropriate or inappropriate to the scenario set-up. Children in both age groups protested specifically against inappropriate acts, indicating they were able to not only follow pretence stipulations and act in accordance with them but to understand their deontic implications. This effect was more pronounced in the 3-year-olds than in the 2-year-olds. The results are discussed in the broader context of the development of social understanding and cultural learning. (Contains 1 figure and 5 footnotes.)