The paper is devoted to one of the topical problems of contemporary linguistics, specifically to the children‘s speech researches and peculiarities of the formation of inner lexicon and lexical-thematic groups in it. It presents the analysis of a fragment of the free associative experiment with speaking children and Ukrainian-Russian bilinguals of young preschool age (that is period of age from three to five). The experiment took place in the Eastern part of Ukraine, precisely Kharkiv as well as in the Western part of the country – in Lviv. The total number of respondents is 416. This number is made of 208 respondents from Kharkiv (104 girls and 104 boys) and 208 respondents from Lviv (104 girls and 104 boys). In the paper we dwell on the issue of lexical-thematic constituents of inner lexicon of a child of young preschool age. The associative fields received in the experiment gave us an opportunity to form lexical-thematic groups of inner lexicon of Ukrainian speaking children and Ukrainian-Russian bilinguals of young preschool age. The analysis showed that there are 25 main lexical-thematic groups: «people», «proper names», «flora and fauna», «elements and nature phenomenon», «colours», «game, toys, sport games», «drinks, food and actions and characteristics connected with them», «alphabet, letters, sounds», «names of months, seasons and derivatives», «study», «clothes, accessories», «location», «direction of movement», «designation of movement», «transport and its components», «feeling of love», «house and its part», «quantity, size», «evaluation», «speech processes», «temperature state», «cognitive processes, memory», «words, that means sounds or their realization», «characters of fairy tale, myths and children‘s stories», «imperative». Taking into account the peculiarities of the problem we compared the results of our analysis with the lexical-thematic groups that were singled out by other researches basing on analysis of inner lexicon of children of other age groups. There were mentions and named the examples of the language norm‘s breaking as a result of impact of bilingualism onto a child‘s language and speech. The reactions that were received in our free associative experiment indicate the state of recipients as active language personalities. Different fragments of the world are in the sphere of their perception. The characteristics of the fragments are realized according to different parameters: time, space, movement, people‘s relationships, quantitative and quality characteristics of the objects. The conclusions made the necessity of further research of the other age group‘s inner lexicon and lexical-thematic groups in it obvious. Key words: children‘s speech, free association experiment, lexical- thematic groups, inner lexicon, monolingual, bilingual.