This paper discusses the variation of Indonesian first person singular pronouns which is related to the acquisition of sociolinguistic competence. The question to be addressed in this study is: to what extent the pronouns are used by Indonesian-speaking children? This study aims to describe the use of saya and aku by children in formal and informal situation. The main data of this study is children's speech, obtained from individual interviews in formal and informal situations. Participants in this study are kindergarteners (N=28) and elementary school children (N=29) whose school is situated in South Tangerang. It is found that there are several variations of self-reference in children's utterances, including saya and aku. It is also found that children in this study are more familiar with aku than with saya. It is also indicated that the children are still learning to use appropriate pronouns in appropriate situations.