1891 census shows Henrietta as a widow, and youngest son aged 4, so at least you know that Robert Johnson died between 1886 & 1891
I think this is probably Robert's death registration, the age and district fits
Jun 1890 Weardale 10a 162 JOHNSON Robert aged 43
and Henrietta's death reg:
Mar 1902 Weardale 10a 168 JOHNSON Henrietta aged 50
Dec 1887 Weardale 10a 154 JACKSON John Wren V aged 81
Jun 1888 Weardale 10a 150 JACKSON Dorothy aged 72
Barbara