You are interested in studying topics such as code-mixing, code-switching, children’s language and secret languages. Which of the following disciplines you need to study for these?

The correct answer is Sociolinguistics.

To understand why this is the correct discipline, let us break down the topics mentioned in the question:
1. Code-mixing and Code-switching: These terms refer to the practice of alternating between two or more languages, or language varieties, in the context of a single conversation or utterance. This behavior is deeply influenced by social factors, identity, and the community of speakers.
2. Children’s language: While the biological aspects of language acquisition fall under psycholinguistics, the study of how children acquire communicative competence, social registers, and adapt their speech to different social settings is a key area of study in sociolinguistics.
3. Secret languages: These are specialized language systems (like argots, slangs, or ludlings) developed by specific groups to prevent outsiders from understanding them. They serve social functions, such as reinforcing group identity and solidarity.

Why Sociolinguistics?
Sociolinguistics is the descriptive study of the effect of any and all aspects of society, including cultural norms, expectations, and context, on the way language is used, and the effects of language use on society. Since all the topics listed (code-mixing, code-switching, social aspects of children's language, and secret languages used by subcultures) deal directly with the relationship between language variation and social context, Sociolinguistics is the primary discipline required to study them.