We present a novel gestural interface for an educational 3D construction game that helps children practice and learn spatial reasoning skills. Previous research shows that having well-developed spatial reasoning skills is crucial to the success in the STEM fields. Our game requires the player to create a number of 3D target objects by moving, rotating, and assembling smaller pieces in the right way through a gestural interface. The gestures were designed with enhancing the user experience and effectiveness of learning in mind, by having a congruent mapping between hand gestures and spatial operations. The initial results of the preliminary study we conducted with the children show that the interface has the potential to be used for practicing spatial reasoning skills with the game. We also discuss how the study can lead to the development of a theoretical framework for designing gestural interfaces for educational games that leverage the benefit of embodied interactions. Copyright is held by the owner/author(s).