Children’s attributions during challenge have roots in toddlerhood with the onset of self-evaluative emotions and may reflect sex differences in early socialization practices. This study examined toddler sex as moderating associations between mothers’ (n = 104, 73.1% White, 75% with a college degree or more) characteristics (low emotional awareness, emotion coaching beliefs) and toddlers’ (Mage = 26.4 mos.; 51 girls) self-evaluative negative attributions of difficulty during a challenging lab task. Negative attributions reflected self-directed comments such as “I can’t do it.” Girls made more negative self-attributions in the context of mothers’ lower emotional awareness whereas boys made more negative self-attributions in the context of mothers’ greater emotion coaching (medium effects). Results demonstrate early sex differences in toddlers’ negative self-attributions to challenge.