The ability to select an appropriate motor response by resolving competition among alternative responses plays a major role in cognitive performance. fMRI studies suggest that the development of this skill is related to the maturation of the frontal cortex that underlies the improvement of motor inhibition abilities. However, fMRI cannot characterize the temporal properties of motor response competition and motor activation in general. We studied the development of the time course of resolving motor response competition. To this end, we used the lateralized readiness potential (LRP), an ERP measure, for tracking correct and incorrect motor cortex activation in children in real time. Fourteen children and 14 adults took part in an animal-size Stroop task where they selected between two animals, presented simultaneously on the computer screen, which was larger in real life. In the incongruent condition, the LRP detected stronger and longer lasting incorrect response activation in children than in adults. LRP results could explain behavioral congruency effects, the generally longer RT in children than in adults and the larger congruency effect in children than in adults. In contrast, the peak latency of ERP waves, usually associated with stimulus processing speed, could explain neither of the above effects. We conclude that the development of resolving motor response competition, relying on motor inhibition skills, is a crucial factor in child development. Our study demonstrates that the LRP is an excellent tool for studying motor activation in children.