The role of metacognitive postdiction accuracy and autonomous and controlled motivation in mathematics was explored in elementary school children (n = 208) within two perspectives, related to sample characteristics. A first study was set up in a population-based cohort. A second study was set up with children with and without a documented mathematical disability. Both studies revealed a concurrent relation between the metacognitive postdiction skills of children and their mathematical accuracy and speed, leading to the practical recommendation that teachers should pay attention to the accuracy of self-judgments of children. In addition, controlled motivation was negatively related to the speed and accuracy in study 2. Children with mathematical learning disabilities (MLD) differed from peers without mathematical learning disabilities on postdiction accuracy and autonomous motivation. However, they did not differ significantly on controlled motivation, suggesting the importance of differentiating between controlled and autonomous motivation when analyzing motivation in mathematics education.