To assess the processing stages involved in attention shifting and response selection tasks in children, we recorded event-related potentials (ERPs) and performance measures during a variant of the Posner paradigm. Subjects responded to visual targets, either preceded by a spatial cue (valid = same side; invalid = opposite side) or presented uncued. Valid targets evoked high-amplitude P1 responses, single-peaked P3s, and the shortest reaction times (RTs). Invalidly cued stimuli evoked delayed RTs, resulting in part from incorrectly oriented attention (decreased P1) leading to delayed target processing (belated N2-P3). Invalid targets also evoked a positive slow wave attributed to prolonged response selection due to cue/target incompatibility. Uncued stimuli elicited the longest RTs, unexplained by deficits in target detection or response selection, which likely resulted from a deficit in motor preparation due to the lack of warning signal. This method may be applied in clinical settings to disentangle selective processing deficits in target detection, response selection, or motor preparation stages.