Young children enforce social norms from early on, but little research has examined how this enforcement behaviour emerges. This study investigated whether observing an adult's norm enforcement influences children's own enforcement of that norm compared with observing an action demonstration without enforcement. Additionally, children experienced enforcement either following their own (second-party) or a third-party's transgression (N = 120). Results revealed that observing enforcement increased two- and three-year-old children's protest against the sanctioned action regardless of second- or third-party context. However, only three-year-olds generalized their enforcement to a novel action not matching the norm, whereas two-year-olds only protested against the previously sanctioned action. Importantly, without any enforcement demonstration, two-year-olds rarely protested at all while three-year-olds did so quite frequently. Thus, providing an opportunity to imitate enforcement seems to give rise to enforcement behaviour in two-year-olds while three-year-olds already understand normative implications following a variety of cues and even apply norm enforcement without any demonstration of how to do it. Statement of contribution What is already known on this subject? Children conform to social norms from early in development. Young children from 2 to 3 years of age also enforce social norms on third parties. What does this study add? Observing enforcement by an adult increases two- and three-year-olds' protest against the sanctioned action. It does not matter whether children experienced enforcement on their own or a third party's action. Three-, but not two-year-olds, generalize their enforcement to novel actions that do not match the norm.