In this functional magnetic resonance imaging study, 17 children were asked to make numerical and physical magnitude classifications while ignoring the other stimulus dimension (number–size interference task). Digit pairs were either incongruent (3 8) or neutral (3 8). Generally, numerical magnitude interferes with font size (congruity effect). Moreover, relative to numerically adjacent digits far ones yield quicker responses (distance effect). Behaviourally, robust distance and congruity effects were observed in both tasks. Imaging baseline contrasts revealed activations in frontal, parietal, occipital and cerebellar areas bilaterally. Different from results usually reported for adults, smaller distances activated frontal, but not (intra-)parietal areas in children. Congruity effects became significant only in physical comparisons. Thus, even with comparable behavioural performance, cerebral activation patterns may differ substantially between children and adults.