"... If this is them in Milwaukee, then there is probably a death certificate for daughter Elizabeth Knapp somewhere between 1870-1880. Any and all help appreciated."
You've probably already found out that there likely isn't a death certificate for daughter Elizabeth Knapp, d. 1870-1880, unless a coroner was involved. But there would be a burial event listed in some church register. Those are generally not the same as a cemetery record. If you find the burial record in the church register, then it's worth checking for a baptism record too.