The thesis is based on implementing a child-centered design approach in the creation of a game authoring tool for children to be used in the classroom as one of their learning tools. Indeed, research in designing such educational software is primarily focused on the learning outcomes, but not on the usability. However, the usability of such software for children is a prerequisite for any learning to be useful and significant. The thesis purposes with a belief that substantial improvements can be made by paying careful attention to usability during the design of the game authoring tool. Unlike the related approaches, the scope of research focuses on the game authoring tool which emphasizes on the design of the game authoring tool and concentrates on the flexibility of the interaction and construction activity in making games. This research methods started by investigating works from other researchers including comparing the usability guidelines used in other fields and exploring the existing educational programming tools for children. The findings exposed the possibility of planning and designing guidelines for usability and limitations or problems faced in these tools. The outcomes were also used to guide the first draft of usability guidelines. To demonstrate and validate these guidelines, a preliminary study was conducted. The study consists of two user studies conducted in two different locations with the goal, to get as much information as possible in children preferences and needs based on the usage of two given software. Informed by these results, the prototype of a game authoring tool named Gatelock was designed. There, design features were used – game programming, game designing and game playing. In game programming, the visual programming method is being carried out. The method is being implemented in a variety of application software for children, especially in the field of interaction design for children. In game design, children use the basic elements of game design in their game making, including planning the games, designing, building, gameplay testing, and re-design. In game playing, the children can participate in the design processes with learning material and they can incorporate elements of various lessons such as physics, mathematics, science and drawing. Throughout the design and development of Gatelock, formative and summative evaluations were conducted to examine the usability and fun aspects of Gatelock. This thesis is a contribution to the scientific domains of child-centered design. It draws on ideas and techniques from the fields of children and interaction design, usability study, game design, as well as evaluating software for children. With respect to game design and children, the main contribution of the thesis can be specified as product-based and process-based. Product-based contributions include a prototype (Gatelock) which has a unique set of features due to its child-centered design, and a list of usability guidelines for designing a game authoring tool for children. Process-based contributions include a preliminary study of creating a new game authoring tool with usability as the first objective, as well as empirical evidences characterizing the preliminary study with results made by children, formative and summative studies which demonstrate the usability of the game authoring tool, empirical evidences on the usefulness of methods, tools and techniques used for the evaluating process with children, and a list of usability guidelines for designing a game authoring tool for children. This thesis also presents the results of the research with the presence of usability guidelines. At the end, both, the game authoring tool and the usability guidelines are iteratively involved in the design refinement.