Both adults and children learn through feedback which events and choices in the environment are associated with higher probability of reward. This probability reward-learning ability is thought to be supported by the development of fronto-striatal reward circuits. Recent developmental studies have applied computational models of reward learning to investigate such learning in children. However, there has been limited development of task tools capable of measuring the cascade of neural reward-learning processes in children. Using a child-version of a probabilistic reward-learning task while recording event-related-potential (ERP) measures of electrical brain activity, this study examined key processes of reward learning in preadolescents (n=30), namely: (1) reward-feedback sensitivity, as measured by the early reward-related frontal ERP positivity, (2) rapid attentional shifting of processing toward favored visual stimuli, as measured by the N2pc component, and (3) longer-latency attention-related responses to reward feedback as a function of behavior strategies (i.e., Win-Stay-Lose-Shift), as measured by the central-parietal P300. Consistent with our prior work in adults, the behavioral findings indicate that preadolescents could learn stimulus-reward outcome associations, but at varying levels of performance. Neurally, poor preadolescent learners (those with slower learning rates) showed greater reward-related positivity amplitudes relative to good learners, suggesting greater reward sensitivity. We also found attention shifting towards to-be-chosen stimuli, as evidenced by the N2pc, but not to more highly rewarded stimuli. Lastly, we found an effect of behavioral learning strategies (i.e., Win-Stay-Lose-Shift) on the feedback-locked P300 over the parietal cortex. These findings provide novel insights into the key neural processes underlying reinforcement learning in preadolescents.