Posters encouraging handwashing would seem to offer a low-cost solution addressing barriers to handwashing in schools. However, what barriers can be successfully addressed and, how effective posters targeted at children may be is not known. In this study, using a co-design methodology, seventy-nine children (aged 6 to 11) from three English schools evaluated and generated handwashing messages in two workshops.The results were then compared with an evaluation (by the authors) of handwashing posters targeted at children. Messages that children considered most effective addressed barriers relating to reminders and encouragement, and education and information (particularly germ transmission, consequence, location and avoidance).Messages that addressed time and social norms were not considered as effective.Posters targeted at children also used reminders and encouragement, and education and information messages. However, the focus of these education and information messages was on instruction (how and when to wash hands), not on germs. Unlike the posters targeted at children, the majority of children's messages were persuasive in that they did more than simply instruct. This has implications for the design of posters and educational material in handwashing interventions.