Using a multiple-deviant oddball paradigm, this study examined second graders' brain responses to Cantonese speech. We aimed to address the question of whether a change in a consonant or lexical tone could be automatically detected by children. We measured auditory mismatch responses to place of articulation and voice onset time (VOT), reflecting segmental perception, as well as Cantonese lexical tones including level tone and contour tone, reflecting suprasegmental perception. The data showed that robust mismatch negativities (MMNs) were elicited by all deviants in the time window of 300-500ms in second graders. Moreover, relative to the standard stimuli, the VOT deviant elicited a robust positive mismatch response, and the level tone deviant elicited a significant MMN in the time window of 150-300ms. The findings suggest that Hong Kong second graders were sensitive to neural discriminations of speech sounds both at the segmental and suprasegmental levels.