There are some half-a-dozen letters, all pretty much the same, no one remembers the Kenyan having a wife or a child, and if he did get some local girl pregnant, he successfully kept that information from his student associates. It wouldn't have been difficult, he lived in one room with a small hotplate for cooking, she lived with her parents. WE just don't know her surname. She shows up shortly after the birth of her child in Seattle, and calls herself Anna Obama. And Mary babysits, according to Mary's story, in January 1961.
That this episode didn't find its way into 'Dreams From My Father' shouldn't surprise anyone. Ann/Anna Obama wasn't meant to be found. And she wasn't Stanley Ann Dunham, who, in January 1961 would have still been in High School somewhere when the child Mary babysat was conceived.
Most of what one needs to find is freely available on THE OBAMA FILE