Whether children attribute guilt to bystanders or evaluate their inaction as wrong when witnessing antisocial behavior is unclear. Therefore, this study examined guilt and moral evaluation related to bystanding in situations involving antisocial behavior among preschoolers (kindergarteners, 5-6 years old) and school-aged children (students in the third and sixth grade). Eighty Japanese participants evaluated the extent of guilt and moral wrongness attributed to classmates who passively observed a moral or conventional transgression in a classroom setting. For comparison, additional scenarios depicted the classmate as being outside the classroom and, therefore, unaware of the transgression. The results indicated that the participants attributed a greater level of guilt to the classmate in the classroom-bystander condition than to the one in the outside-the-classroom condition. However, the guilt ratings did not differ significantly across age groups or types of transgression (moral vs. conventional). Regarding moral evaluation, the classmate in the classroom-bystander condition was judged more negatively than the one in the outside-the-classroom condition, and moral transgressions received harsher evaluations than conventional ones. Moreover, although no age-related differences emerged in the outside-the-classroom condition, students in the third and sixth grade judged the bystander more negatively than preschoolers in the classroom-bystander condition. Based on the observed correlation between guilt and moral judgment, these findings suggested that the two constructs become increasingly interconnected with age. Furthermore, while young children often make judgments without considering contextual factors, such as the type of act and the presence of bystanders, older children increasingly incorporate these factors into their evaluations.