There is ever-increasing global and local attention to children’s participation rights. As activities have proliferated, so have concerns about children’s participation having an impact on decision-making. This article looks to what can be learned conceptually and practically from children’s activism, which have examples where children have actively changed decisions in their communities. This article examines one such example, where groups of children in Bangladesh stopped the illegal marriages of children. Research was undertake with two Child Forums, involving 36 child activists. Further interviews were undertaken with girls whose marriages had been stopped and adults who had key roles in the child activism, such as local police officers and civil servants. The analysis finds that: (1) the legal context was critical to the child activists’ success, not because parents were arrested but because children could mobilise local officials; (2) the activism was collective rather than individualistic, supported by a nexus of relationships; (3) successes depended on children’s persistent and urgent activity, in order to mobilise other people’s attention and actions. The article concludes that the children’s participation field should attend to the conceptual lessons from activism and its practical contributions in: recognising the critical importance of the ‘shadow of the law’; building children’s political capital, and supporting the spaces and time for children to mobilise alongside educational demands.