Final answer:
The subject pronoun 'tú' is used to address a child in Spanish.
Step-by-step explanation:
The subject pronoun that you would use to address a child in Spanish is tú. It is informal and used when speaking to someone of the same age or younger. For example, you would say 'Tú eres inocente' to mean 'You are innocent' when addressing a child.
Learn more about Subject pronouns in Spanish