Beliefs about child competence in math and reading have important implications for academic performance in adolescence. However, it is unclear whether children’s own beliefs are the most important predictor of their academic performance or whether parents’ and teachers’ beliefs about child competence influence child academic performance. We assessed mothers’, fathers’, teachers’, and children’s beliefs about European American children’s (N = 189) competence in math and reading at age 10 and children’s math and language performance at ages 10, 13, and 18 years. Confirmatory factor models demonstrated that children’s and teachers’ beliefs had lower loadings on a latent variable of child competence in math and reading than mothers’ beliefs. Children’s self‐competence beliefs in math and reading were not significantly correlated, suggesting children may use dimensional comparisons when assessing their own competence. Mothers’, fathers’, and teachers’ assessments of child competence in math were strongly correlated with their assessments of child competence in reading. Controlling for stability in academic performance, family socioeconomic status, and other reporters, mothers and fathers who rated their children’s math competence higher had adolescents who performed better in math, and fathers who rated their children’s reading competence higher had adolescents who performed better in language tasks. However, children who rated their own competence higher in math and reading had lower math and language (for girls only) performance in adolescence. European American children may use dimensional comparisons that render them poorer judges of their math and reading competence than parents.