Accurate monitoring and control are essential for effective self-regulated learning. These metacognitive abilities may be particularly important for developing math skills, such as when children are deciding whether a math task is difficult or whether they made a mistake on a particular item. The present experiments investigate children's ability to monitor and control their math performance. Experiment 1 assessed task- and item-level monitoring while children performed a number line estimation task. Children in 1st, 2nd, and 4th grade (N = 59) estimated the location of numbers on small- and large-scale number lines and judged their confidence in each estimate. Consistent with their performance, children were more confident in their small-scale estimates than their large-scale estimates. Experiments 2 (N = 54) and 3 (N = 85) replicated this finding in new samples of 1st, 2nd, and 4th graders and assessed task- and item-level control. When asked which estimates they wanted the experimenter to evaluate for a reward, children tended to select estimates associated with lower error and higher confidence. Thus, children can accurately monitor their performance during number line estimation and use their monitoring to control their subsequent performance.