Mobile augmented reality (mobile AR) enables virtual content such as 3D models, animations and annotations to be placed on top of a real world objects in any context. We applied mobile AR to develop the Calory Battle AR exergame to tackle worldwide childhood obesity. In this game the player finds and defuses virtual calory bombs in a real world environment. Specifically, we present the development of two game versions. First prototype was created without a third party game engine and it led to many challenges. To explore solutions to these challenges, we created a new version of game with the Unity 3D game engine. Using the Unity 3D, the game development process was simplified. A mixed-method usability evaluation on children and university students indicated that especially interaction with AR content and user interface clarity were improved in the Unity 3D version. This study produced three important contributions: 1) a novel mobile AR exergame to motivate children to move; 2) reimplementation of the game using the Unity 3D; and 3) results of a usability evaluation comparing two game versions. We expect that game engines such as the Unity 3D will become essential for AR game development in the future.