As we emailed, Joseph is age 1 in the 1880 census in Iowa and not found in the 1885 census, thus he likely died before 1885.
I haven't found the little girl.I agree that she probably was born and died between census years. That could be in the long stretch between cenuses (from 1885-1900) or at any other time in the life of the family, if she died as an infant or quite young.
Her birth and death may be registered at the Allamakee County, Iowa, courthouse. IAgenweb may have volunteers who would be willing to do a look up.