In this paper, we present the FUBImethod, a method based on six stages to co-design interactive experiences based on Full-Body Interaction. The FUBImethod aims specifically to engage children in co-design processes and to benefit from their natural playfulness and expertise in movement. This approach allows designers to go beyond the surface level of content-driven ideas by raising awareness of the body and space, and by proposing techniques that help the design team to understand and incorporate the specific qualities that constitute Full-Body Interaction. We also propose strategies to strengthen the children's perspective in the design process in reaching a common agreement in the design goals and the selection of adequate design choices within the design team. We ground our theoretical discussions on the outcomes of the research project "Evaluation-Driven Design", the goal of which was the exploration and definition of adequate research and design methods in Full-Body Interaction.