Cognitive effects on linear sagittal vection in children were investigated. Forty children (7 and 11 years old) were exposed to a bilateral backward optical flow in a single physical condition (seated in a stationary armchair) but in two contrasted cognitive conditions. In one cognitive condition, the children were precisely informed that the armchair could move. In the other, they were informed that the armchair could not move. In each age group, half the children were assigned to one cognitive condition, the other half to the other condition. The results indicate that knowledge about the plausibility of a physical displacement does not affect the probability of obtaining vection. However, at both ages, the latencies for reporting vection were shorter when the physical displacement was known to be possible than when it was known to be impossible. The present results indicate that exclusively cognitive factors do not affect vection occurrence but can modulate latencies for reporting vection.