Despite increases in visibility, gender-nonconforming young people continue to be at risk for bullying and discrimination. Prior work has established that gender essentialism in children correlates with prejudice against people who do not conform to gender norms, but to date no causal link has been established. The present study investigated this link more directly by testing whether children's gender essentialism and prejudice against gender nonconformity can be reduced by exposure to anti-essentialist messaging. Children ages 6--10 years of age (N = 102) in the experimental condition viewed a short video describing similarities between boys and girls and variation within each gender; children in the control condition (N = 102) viewed a corresponding video describing similarities between two types of climate and variation within each. Children then received measures of gender essentialism and prejudice against gender nonconformity. Finally, to ask whether manipulating children's gender essentialism extends to another domain, we included assessments of racial essentialism and prejudice. We found positive correlations between gender essentialism and prejudice against gender nonconformity; both also correlated negatively with participant age. However, we observed no differences between children in the experimental versus control conditions in overall essentialism or prejudice, indicating that our video was largely ineffective in manipulating essentialism. Accordingly, we were unable to provide evidence of a causal relationship between essentialism and prejudice. We did, however, see a difference between conditions on the discreteness measure, which is most closely linked to the wording in the video. This finding suggests that specific aspects of essentialism in young children may be modifiable.