When visual information is available, human adults, but not children, have been shown to reduce sensory uncertainty by taking a weighted average of sensory cues. In the absence of reliable visual information (e.g. extremely dark environment, visual disorders), the use of other information is vital. Here we ask how humans combine haptic and auditory information from childhood. In the first experiment, adults and children aged 5 to 11 years judged the relative sizes of two objects in auditory, haptic, and non-conflicting bimodal conditions. In "Experiment 2", different groups of adults and children were tested in non-conflicting and conflicting bimodal conditions. In "Experiment 1", adults reduced sensory uncertainty by integrating the cues optimally, while children did not. In "Experiment 2", adults and children used similar weighting strategies to solve audio-haptic conflict. These results suggest that, in the absence of visual information, optimal integration of cues for discrimination of object size develops late in childhood.