Participatory approaches to engaging in research with young children place a great deal of emphasis on children's rights to choose whether or not they wish to be involved. A number of recent studies have reported a range of strategies both to inform children of their research rights and to establish options for checking children's understanding of these rights throughout the research process. This paper seeks to move the debate around children's informed agreement to participate forward by considering the ways in which children might indicate their dissent--their desire not to participate--at various stages of the research process. Drawing on examples from Iceland and Australia, involving children aged two-six years, the paper explores children's verbal and non-verbal interactions and the ways in which these have been used, and interpreted, to indicate dissent. Reflection on these examples raises a number of questions and identifies several tensions, as well as offering some suggestions for ways in which researchers can recognise children's decisions to opt out of research participation. (Contains 1 figure.)