Sustained selective attention is a crucial component of many higher-order cognitive processes; yet there is little research into the mechanisms of this ability early in development. One of the challenges in investigating mechanisms of sustained selective attention in young children is lack of appropriate experimental paradigms. This paper reports findings from a novel paradigm designed to investigate mechanisms of sustained selective attention in young children - the Object Tracking task. Results of two experiments with 3- to 5-year-old children provided support to the notion that development of the endogenous component of selective sustained attention lags behind the development of the exogenous component of this process. Importantly, the Object Tracking paradigm allowed investigating both of these components within the same task, thereby making it possible to attribute changes in performance to different mechanisms of attentional control rather than to differences in the level of motivation and engagement in different tasks.