Is it permissible, or rather a lie, to teach a boy to address a man as "my uncle" (ʿammī) and a woman as "my aunt" (khāltī) out of politeness, knowing that they are not actually so?

The child's guardian must teach them to respect elders and relatives. Allah has considered the paternal uncle as a father, and the Prophet, may Allah bless him and grant him peace, called his paternal uncle Al-Abbas "the remainder of his fathers." Therefore, there is no religious impediment for a young person to address an elder as "O uncle" or "O aunt." This falls under the category of good manners and respect for elders.