Compared to their neurotypically developing peers, children and adolescents with autism spectrum disorders tend to have attenuated neural responses in the parietal lobe when attending sensory input, as reflected by a reduced P3b amplitude measured with electroencephalography. However, it is unknown whether a reduced P3b amplitude in autistic children and adolescents is associated with their autism traits, daily functioning, and/or cognitive functions. To address these questions, we assessed 57 children with autism aged 7-14 years and 57 typically developing children with electroencephalography using a binaural auditory oddball paradigm. Participants further underwent cognitive assessment, and parents reported on autistic traits, executive functioning, and adaptive functioning. As expected, children with autism had lower P3b amplitude compared to controls. Across all participants, a lower P3b amplitude was associated with more parent-reported social-communication problems and impairments in daily executive functioning. Children with autism had reduced visual processing speed, which was coupled to a lower P3b amplitude. In conclusion, we found attenuated P3b amplitude in children with autism performing an auditory selective attention task, which generalized to impaired cross-modal processing of visual input and an underlying impairment in allocating attentional resources critical for social and executive functioning.