Children show less positivity toward gender-nonconforming (GN) than gender-conforming (GC) peers. Yet, little is known about children's reasoning about peers of varying gender expressions, including age-, gender-, and culture-related influences. We investigated how children aged 4- to 5- and 8- to 9-years-old in Hong Kong and Canada (N = 678) reason about their moral judgments of GC and GN peers. After viewing vignettes describing GC and GN boys and girls, we asked children whether each target peer's behavior was right or wrong and why they thought so. We coded children's reasoning using a new coding scheme developed via inductive content analysis. Overall, children's most commonly used reasoning styles were global standard, personal choice, gender stereotypes, "don't know," and others' welfare. Children used more gender stereotype-related reasoning when they were older and from Hong Kong, appraising the GN boy, or when they perceived the target's behavior as wrong. In contrast, children reasoned based on personal choice more when they were from Canada or when they perceived the target's behavior as right. These findings inform how age-, gender-, and culture-related factors are associated with children's reasoning about the acceptability or appropriateness of varying kinds of childhood gendered behavior. They provide insights regarding children's appraisals of different gender expressions by illuminating not only how they view GC and GN peers but also, from their own perspectives, why they do so. These insights have implications for strategies aimed at decreasing gender-related biases and increasing children's acceptance of gender diversity.