The ability to flexibly switch between tasks develops during childhood. Children’s task-switching performance improves with practice, but the underlying processes remain unclear. We examined how nine weeks of task-switching training affect performance and task-related activation and connectivity as assessed by functional magnetic resonance imaging. Children (8–11 years) were pseudo-randomly assigned to three groups: high-intensity task switching (SW; n = 70), high-intensity single tasking (SI; n = 72), and passive control (n = 41). After three weeks, drift-diffusion modeling revealed faster evidence accumulation and more cautious responding in both training groups relative to the control group. At the end of training, these changes were maintained in the SW group only, that also showed activation decreases in dorsolateral prefrontal cortex. Functional connectivity increases associated with task-switching demands became less pronounced with practice in both training groups, with more persistent decreases in the SI group. We conclude that task-switching training altered performance by accelerating evidence accumulation and promoting more cautious responding. Faster evidence accumulation along with decreased task-related activations suggest increased processing efficiency in frontoparietal regions with training. More intense task-switching training helped maintain these changes, possibly by facilitating plastic change through the protracted mismatch between processing supplies and environmental demands.