The ability to recognize and evaluate reliable informants is a critical skill for effective social learning. Building on prior work showing children's sensitivity to informants who omit relevant information, here we asked whether children's teacher evaluations incorporate information about 1) the epistemic state of the teacher, and 2) the amount and value of information taught. Preschool-aged children rated informants who taught learners about a novel toy with four functions; we systematically varied the number and value of functions the teachers knew and taught. Our results indicate that children exonerated unintentional omissions of teachers who had incomplete knowledge, and provided graded ratings based on the degree of omission. These findings are consistent with the predictions of prior computational work, and suggest that the ability to reason about others' knowledge plays an important role in children's inferences about others' efficacy as informants.