Background: Infrequent deviants in a rapid sequence of sounds elicit a negative cortical potential over the frontocentral midline (mismatch negativity, MMN) followed by a positive deflection (P3a). Both cortical potentials are consistently attenuated in patients with schizophrenia (SZ), and, to a lesser degree, in patients with bipolar disorder (BP). Objective: Since it is unclear when MMN and P3a deficits arise relative to the emergence of symptoms, we examined whether MMN and P3a alterations are already detectable in children with familial high risk. Methods: Using 128-channel electroencephalography, we recorded auditory MMN and P3a evoked by a deviation in sound duration, frequency, or both in 51 children with familial high-risk for SZ (FHR-SZ), 41 children with familial high-risk for BP (FHR-BP), and 39 population-based children (PBC) at a mean age of 12.10. Results: MMN amplitude evoked by a duration deviant was larger in children with FHR-BP compared to PBC and FHR-SZ. P3a amplitude in response to a duration ∗ frequency deviant was larger in children with FHR-BP compared to children with FHR-SZ, but not compared to PBC. MMN- and P3a-peak latency did not differ between groups. Conclusions: At an age of around 12 years, children with FHR-BP display enhanced neural sensitivity to change detection of duration deviants, while FHR-SZ showed a normal response pattern. Longitudinal recordings in high-risk children during adolescence are required to elucidate the temporal trajectories of MMN and P3a responses and how they relate to the emergence of first clinical symptoms in SZ and BP.