In this paper, children's mental activities when making digital games are explored. Where previous studies have mainly focused on children's learning, this study aimed to unfold the children's thinking process for learning when making computer games. As part of an ongoing larger scale study, which adopts an ethnographic approach, this research reports on how children think when making their own computer games using their "Thinking Maps" and video recordings of group discussions. A model for Thinking for Learning, "think-to-learn-to-think," is discussed and children's activities are used to define the stages of this cycle. The study of children's pre-during-after project "Thinking Maps" and explanations during group discussions suggests that their thinking sequences were altered during and after their game design activities. The children presented their "thinking process" when designing computer games as a continual circular cycle and described their mind as a virtual lab to plan and visualise solutions before testing them on screen, using the "Alice" game design program.