think this is the son frederick in 1881
RG11/297/11/16
jane.e.mckay age 44. b. southampton. occ. mantle maker(wid)
fred.j.mckay age 14. b. st.pancras. occ. errand boy. son
living at;8,brett road,hackney
so if the alexander in 1871 is him and this is as i beleive his wife and son
then he did die between 1871-1881