We investigated children's moral behaviour in situations in which a third party was harmed (the test case for possession of agent-neutral moral norms). A 3-year-old and two puppets each created a picture or clay sculpture, after which one puppet left the room. In the Harm condition, the remaining (actor) puppet then destroyed the absent (recipient) puppet's picture or sculpture. In a Control condition, the actor acted similarly but in a way that did not harm the recipient. Children protested during the actor's actions, and, upon the recipient's return, tattled on the actor and behaved prosocially towards the recipient more in the Harm than in the Control condition. This is the first study to show that children as young as 3 years of age actively intervene in third-party moral transgressions.