By 3 years of age, children tattle about rule violations they observe, even as unaffected bystanders. It is argued that tattling is one way in which children enforce norms and that in the long term, it helps sustain co‐operation (e.g., Vaish, Missana, & Tomasello, 2011). However, an alternative explanation could be that children are worried that the victim might blame them and so feel the need to inform the victim about who caused the harm. The present study aimed to tease these possibilities apart. Children observed a puppet either causing harm to another puppet (e.g., destroying their artwork) or no harm (e.g., destroying a different object). Importantly, the situation was constructed such that children knew they could not be blamed for the transgressions. Nonetheless, 3‐year‐old children tattled on the transgressor more when the transgressor had caused harm than no harm. Thus, young children's tattling about third‐party moral transgressions seems to be aimed at enforcing norms. An additional, exploratory goal of this study was to examine the relation between children's temperament and norm enforcement. Temperamental shyness negatively correlated with children's protesting and tattling behavior, though more research is needed to better understand the role of temperament in early norm enforcement.