To date, little is known about the neural underpinnings of social-emotional processes in young children. The present study investigated the time course of children's ERP responses to facial expression and personal familiarity, and the effect of these variables on ERP measures of effortful attention in a Go-Nogo task. Dense-array EEG was collected from 48 4-6-year-old children who were presented with pictures of their mothers' and strangers' happy and angry faces. ERPs were scored following face presentation and following a subsequent cue signaling a Go or Nogo response. Responses to face presentation showed early perceptual components that were larger following strangers' faces, suggesting facilitated rapid processing of personally important faces. A mid-latency frontocentral negativity was greatest following angry mothers' faces, indicating increased attentional monitoring and/or recognition memory evoked by an angry parent. Finally a right-lateralized late positive component was largest following angry faces, suggesting extended processing of negatively valenced social stimuli in general. Following the Go-Nogo response cue, a right-lateralized mid-latency negativity thought to measure effortful attention was larger in Nogo than Go trials, and following angry than happy faces, possibly reflecting increased effortful control required in those conditions. The present study suggests that overlapping but differentiated networks for both rapid and elaborative processing of important socio-affective information are established by 4-6 years. Moreover, the extended spatial and temporal distribution of components suggests a pattern of response to social stimuli in which more rapid processes may index personal familiarity, whereas temporally extended processes are sensitive to affective valence on both familiar and unfamiliar faces.