High density diffuse optical tomography (HD-DOT) combines logistical advantages of fNIRS with enhanced image quality, validated extensively against fMRI in adults and neonates. However, HD-DOT is yet to be evaluated in preschool-age children. Here we present an HD-DOT system optimized for preschoolers, including a 128-source by 125-detector console, light-weight fiber optics, and an expanded field-of-view. We validated the system by mapping cortical activations during visual, auditory, and motor tasks in adults. We then imaged children while they watched movies, finding reproducible patterns of brain activity and showing that feature regressors can map functionally specific regions from movie-viewing data in preschoolers.