I just looked and you are right, I am sorry. I have read the Sage biographies before and I don't remember seeing it.
The 1911 census was filled in by John Sage himself, and is signed by him. I doubt he forgot that two children died, so I am wondering if it was a mistake made by whoever wrote his biography.
Just out of interest, I looked for infant deaths in Hackney and in Norfolk, and these are the only ones I found:
Third quarter 1892
Sophia Sage, age 1, Hackney,
First quarter 1896
Dorothy May Sage, age 0, Hackney
Third quarter 1897
Albert Edward Sage, age 1, Hackney
I don't think any of these children are John and Annie's. Annie had children in 1891, 1892, 1894, 1895, 1897, 1899, 1901, 1904 and 1907.
They married in November 1890 so that leaves 1893, 1896, 1898, 1900, 1902, 1903, 1905 and 1906
I can't find deaths for any infants during these years in Hackney or in Norfolk.
If anyone can shed any light, I would be interested to know if these two Sage infants did exist or if it’s a mistake in research. Until I find evidence showing otherwise, I am inclined to believe my own (if quick) research into the matter and also the census document which was filled in by John Sage himself.