Recently, the computer software has become an increasingly important share in the overall industry and many countries have been adapted an early SW education as their mandatory curriculum in school for children. Most of the prior SW education curriculums had been focused on how to learn a programming language to make a SW in a computer, but current SW education goal has been moved onto training the computational thinking to solve various problems with the computer, effectively. The paper exploits a game design activity as a good and fun tool to training such computational thinking for children. The proposed game design activity consists of a series of systematic curriculums to design a creative game in offline for the children who do not have any prior-knowledge of game. Participants first understand game elements such game title, synopsis, rules, game-play, which are indispensable to make a game. Then they try to understand what the fun is in the game and discuss with how to make such fun in the game. Finally, they have proposed a creative game design which is motivated by playing board-style games and iPad-games. With our assessment of the proposed activity, our game activity derives let the participants to computational thinking while creating the synopsis and game rules to define the game. It also revealed increased interest and understanding of computer software. We expect that the proposed game design activity could be an alternative to train computational thinking and a more holistic program to have increased interest and understanding of computer software for children.