Visual tracking of multiple objects in a complex scene is a critical survival skill. When we attempt to safely cross a busy street, follow a ball's position during a sporting event, or monitor children in a busy playground, we rely on our brain's capacity to selectively attend to and track the position of specific objects in a dynamic scene. This ability to visually track simultaneously moving objects in a continuously changing and multisensory environment is a critical component of nearly all forms of visual-motor coordination. While methods for assessing Multiple Object Tracking (MOT) in adults are well established, due to challenges associated with designing a MOT task suitable for young children, we have little understanding of MOT abilities under the age of 5-years. To better understand how and when young children learn to track multiple objects, we designed, implemented and evaluated "TrackFX," the first game-based MOT task running on a touch tablet designed for children as young as 30 months old. We present findings from an empirical study of 31 children between the age of 30 and 58 months and implications for game-based learning. (Contains 7 figures.)