This paper explores how a co-design process can be centred around pre-schoolers' enjoyment of constructive play practices, so that they, rather than adults, become protagonists in a design process. The pilot study was conducted involving 25 children from 3-6 years of age in an intuitive three-step design process that allowed the children to self-reliantly express themselves and make their own design decisions. Generative tools, storytelling, and a set of open-ended design tasks stimulated the pre-schoolers to design tactile 3D shapes. Observational data provided insight in the children's playtime and focus during the design process. The results showed that it is possible for pre-schoolers to (1) establish focus on construction play (environment), (2) that their explorative playing is led through distinctive phases of a design process (activity), and (3) that they are able to construct something specific by playing (concept).