Adults teach children not to "judge a book by its cover." However, adults make rapid judgments of character from a glance at a child's face. These impressions can be modestly accurate, suggesting that adults may be sensitive to valid signals of character in children's faces. However, it is not clear whether such sensitivity requires decades of social experience, in line with the development of other face-processing abilities (e.g., facial emotion recognition), or whether this sensitivity emerges relatively early, in childhood. An important theoretical question therefore, is whether or not children's impressions are at all accurate. Here, we examined the accuracy in children's impressions of niceness and shyness from children's faces. Children (aged 7-12 years, ~90% Caucasian) and adults rated 84 unfamiliar children's faces (aged 4-11 years, 48 female, ~80% Caucasian) for niceness (Study 1) or shyness (Study 2). To measure accuracy, we correlated facial impressions with parental responses to well-established questionnaires about the actual niceness/shyness of those children in the images. Overall, children and adults formed highly similar niceness (r = 0.94) and shyness (r = 0.84) impressions. Children also showed mature impression accuracy: Children and adults formed modestly accurate niceness impressions, across different images of the same child's face. Neither children nor adults showed evidence for accurate shyness impressions. Together, these results suggest that children's impressions are relatively mature by middle childhood. Furthermore, these results demonstrate that any mechanisms driving accurate niceness impressions are in place by 7 years, and potentially before.