In the antebellum and Jim Crow south, black people were virtually never referred to as adults by white people. Young and middle aged adult black people would still be referred to by (at best) their first names and more often as “boy” or “girl” (or possibly some other even less polite words). Because it’s ridiculous to refer to an elderly person as “boy” or “girl,” elderly black people, especially those who had a certain level of status as slaves, would instead be referred to as “uncle” or “aunt.”
The south at that time was highly formal about these things; you’d never refer to an adult white person, even if they were a servant, as “boy” or “girl” in that same way, and first name only was for people you were very familiar with, or maybe children. You’d pretty much always be expected to refer to adults by Mr, Mrs, Miss, Dr, Reverend, etc. Even young children would sometimes be referred to with an honorific (Miss or Master. Note, in this context, “master” does not necessarily mean “slave master,” it’s just a way to differentiate between a boy and an adult man, who would be “mister”). Referring to adult black people with familiar terms like “uncle” or “aunt,” or juvenile terms like “boy” or “girl,” or over familiar terms like their first name, while simultaneously requiring that same person to refer to a white toddler as “Miss Susie” or “Master Jimmy,” means that black people were permanently placed in a subservient child class below actual children. Aunt and Uncle aren’t “child” titles, but still have an implication of extreme familiarity. Your aunt and uncle are people who take care of you. There’s a reason the term Uncle Tom is used to refer to a stupid black person who caters to white people for validation at the expense of other black people and their own dignity.
Uncle Ben and Aunt Jemima were originally conceived as nostalgic characters of the antebellum south. They were implied to be, if not explicitly stated, to be literal slaves who spoke ignorantly and found joy only in cooking for their white “masters” (as in slave owners). They were drawn or acted as shucking, jiving minstrel caricatures of black people. As time went on, those aspects of the characters were obviously dropped, with Uncle Ben looking more like a chef and Aunt Jemima having more of a businesswoman or housewife vibe, but the terms still carry an implication of slavish black servants looking only to cater to white people’s tummies.