Рассматриваются структурные особенности русско-английских переключений кодов в речи двух пятилетних детей, одновременно усваивающих русский и английский языки в русской семье в России. Анализируются переключения кодов трех типов: выбор кода, межфразовые и внутрифразовые переключения. Устанавливается связь структур переключений с особенностями формирования детского билингвизма. Выявлено, что разные типы переключений по-разному отражают усиление несбалансированности в развитии билингвизма детей. This article aims at establishing the interrelation between the structural aspects of mixed speech and the development of two five-year-old siblings’ bilingualism. These boys have been acquiring Russian and English simultaneously in the ‘one parent – one language’ bilingual context in a Russian family in Russia. The authors classify the observed code-switches into three main types: code-choice, intersentential switches, and intrasentential ones. Codechoice demonstrates contacts of two languages only in semantic and pragmatic aspects, thus reflecting various kinds of the bilingual discursive strategy. Intersentential code-switches can shed light on the ways bilingual children structure Russian and English sentences combined within a single utterance. Contacts between grammars are minimal in such subtypes of intrasentential code-switches as clause-switches and parenthetical switches, while in other subtypes (insertions proper, classical switches, island switches, and bare forms) the interrelation is much closer. Structural features of the bilingual utterances of the latter subtypes are analyzed within the framework of the Matrix Language Frame Model (MLFM) elaborated by C. Myers-Scotton. The authors have adopted the MLFM for the analysis of child bilingual utterances. The analysis of each type and subtype of code-switches is combined with the description of the two siblings’ bilingual development. The authors have found out that codeswitches reflect the unbalanced development of child bilingualism in the following ways: (1) frequent code-shifts from English to Russian in dialogs with English-speaking adults; (2) predominant employment of Russian as the Matrix Language; (3) frequent use of English bare forms in Russian morphosyntactic frames; (4) rare occurrences of classical code switches. The mixed speech of the five-year-old children showed that they are competent enough not to violate the principles of the MLFM, with occasional violations of the System Morpheme Principle observed only in English morphosyntactic frames. The comparison of the siblings’ code-switches has revealed that at the age of five they have more common than specific characteristics in all structures. The differences in the structural characteristics of their codeswitches are mainly quantitative: code-switches are more numerous and variable in the younger boy. The results of the research can contribute to the study of simultaneous child bilingual development via structural and pragmatic analysis of code-switches. Moreover, the analysis of child bilingual speech can verify some aspects of language contact theory and cognitive processes underlying production of bilingual individuals’ speech.