Situated Expectancy Value Theory (SEVT) emphasizes the role parents play in shaping children's confidence and value they place on science. Although a robust literature has documented parental influences on children's science motivations in school-based contexts, less is known about how parents' attitudes and support shape children's science motivation in extracurricular learning environments. Moreover, prior work has rarely considered whether motivational processes differ across science domains, such as life versus physical sciences. The present study addressed these gaps by examining whether parents' attitudes and support predicted children's science motivation during participation in a regional science fair, and whether patterns varied by project domain. The study examined 96 parent-child dyads (60% girls; Grades 3-11). Hierarchical regression analyses indicated that parents' perceptions of children's science task value were the most consistent positive predictor of children's motivational beliefs, significantly predicting both ability beliefs and task value. The encouragement children reported receiving from their parents was a positive predictor of children's task value, but parental reports of encouragement were not. In contrast, instrumental support through hands-on help was negatively associated with children's ability beliefs across both parent and child reports. No significant differences were found between life and physical science projects. This research indicates that parents play an integral role in children's science motivation development when they participate in extracurricular science-learning opportunities. Subsequently, parents may need external support to learn how to provide appropriate help on student projects and offer general science encouragement that promotes science motivation.