The Ancestry.com information does not indicate a racial identifier, only gender. It does show that the two servants were an 18 year old Female and a 14 year old Male. Their names appear to be of European origins, but that is not a positive indicater of their status. Their names do not appear to indicate that the Boy and Girl are related.
According to the personal information (contrary to popular mythology) Lincoln was certainly wealth enough in 1860 to have hire a housekeeper/nanny and a valet/groomsman without them being of African origins. But then you still have to factor in that Mary Todd was still from a slave holding family. more suited to being a mistress of the home rather than a housekeeper. And under the laws of the land (a popular phrase now days) they could have been african, or even indentured as David pointed out that Illinois law allowed, rather than free hires.