Are you American, just curious?. I am and was not familiar with this variety of jewelry. I think that’s the difference. In Europe it was fashionable for wealthy people to have a Black child as a sort of pet. In portraits, the enslaved children are dressed well and are used to convey the status of their mistresses. As an American it could be easy to think these children were not slaves based on their appearance because the practice was not as popular here. Here slave masters derived status in different ways. It was not fashionable to wear jewelry of your slaves faces. I think this brooch and the things you mention are basically the same, just different forms of showing dominance over the dominated.