Todd Compton speculates that Fanny Alger may have miscarried.
Todd Compton, In Sacred Loneliness: The Plural Wives of Joseph Smith (Salt Lake City: Signature Books, 1997), 35
Again an unsympathetic voice refers to the relationship as a marriage, a sealing. In addition, Webb acknowledges that this was a fully sexual union. Since there is no record of Fanny having a child, either Webb was mistaken (though this seems unlikely, if Fanny lived in his home after leaving the Smith home), the child was miscarried or died young, or it was raised under another name. Without further documentation, there is no way of knowing.