Multisensory integration of spatial information occurs late in childhood, at around eight years (Gori, Del Viva, Sandini, & Burr, 2008). For younger children, the haptic system dominates size discrimination and vision dominates orientation discrimination: the dominance may reflect "sensory calibration," and could have direct consequences on children born with specific sensory disabilities. Here we measure thresholds for visual discrimination of orientation and size in children with movement disorders of upper limbs. Visual orientation discrimination was very similar to the age-matched typical children, but visual size discrimination thresholds were far worse, in all eight individuals with early-onset movement disorder. This surprising and counterintuitive result is readily explained by the cross-sensory calibration hypothesis: when the haptic sense is unavailable for manipulation, it cannot be readily used to estimate size, and hence to calibrate the visual experience of size: visual discrimination is subsequently impaired. This complements a previous study showing that non-sighted children have reduced acuity for haptic orientation, but not haptic size, discriminations (Gori, Sandini, Martinoli, & Burr, 2010). Together these studies show that when either vision or haptic manipulation is impaired, the impairment also impacts on complementary sensory systems that are calibrated by that one. (Contains 3 figures and 1 video.)