Metacognitive skills have been shown to facilitate learning. But do they develop globally or within content domains? We used an objective measure of metacognition in two different domains to investigate this question in children. 25 subjects (5 to 8 y.o.) made numerosity judgments (which picture has more dots?) and emotion judgments (which picture looks happier?) on a touchscreen. After each judgment, they placed a metacognitive “bet” on their accuracy using a token economy, with immediate feedback (+3/-3 high risk, +1/-1 low risk). We measured metacognition by a phi correlation of risk choice and accuracy, finding that children were significantly metacognitive on both tasks. Higher metacognitive scores on the numerosity task, and not the emotion task, predicted mathematical intelligence. However, metacognitive scores did not predict other measures of ability, such as general IQ. Our study provides evidence that metacognition develops in tandem with domain-specific knowledge, rather than globally.