It was investigated whether task-related body movements yield beneficial effects on children's learning of two-digit numbers and whether these learning effects are affected by mirror-based self-observation of those movements. Participants were 118 first-graders, who were randomly assigned to two movement conditions and two non-movement control conditions. In the movement conditions, children were instructed to build two-digit numbers by making and simultaneously verbalizing out loud different sized steps representing the smaller units the numbers consisted of (e.g., the number "36" was construed by saying out loud "10," "20," "30," "35," "36," while making three big steps, one medium, and one small step) on a ruler across the floor. In one of the movement conditions, the children were additionally asked to observe their steps in a mirror. In the first conventionally taught control condition, the children were asked to verbally build and mark the two-digit numbers on a ruler depicted on a sheet of paper. In the second control condition, children were seated before the ruler across the floor, and after verbally constructing the two-digit number, they had to walk to the appropriate position of the number on the ruler across the floor. In the subsequent test phase, children's knowledge of two-digit numbers was assessed by a final math test. The results confirmed the hypothesis that the movement conditions lead to higher test performance than the non-movement condition and revealed that test performance was not differentially affected by mirror-based self-observation.