Previous research in bilingual adults indicates age of acquisition for a second language has little impact on brain organization for semantic processing. Native speakers, early-learners, and late-learners of a second language all show evidence of a similar N400 in response to semantic anomalies. However, these studies are typically conducted with adults who have both fully mature brains and many years of experience with their second language. Studies with bilingual children provide the opportunity to test the relative impact of maturational age and language proficiency on brain organization for semantic processing. In the current study, event-related potentials (ERPs) were recorded while Spanish-English bilingual and monolingual English-speaking children 6 to 8 years of age listened to naturally spoken English sentences containing semantic anomalies. Bilingual children had been exposed to Spanish since birth and English for the past 2 - 2.5 years. Monolinguals and high-proficiency bilingual children showed typical N400 responses to the semantic violations. However, bilingual children with low proficiency in English did not show this typical effect. Further, the differences in the N400 for bilingual children were not dependent on age of acquisition. These results suggest that brain organization for semantic processing is largely dependent on proficiency and support previous findings that any differences in semantic processing for first and second language learners are mediated by proficiency rather than age of acquisition.