We have a bit of a quandary here and I am going to need all the available information you can send me to try and sort this out for you.
I presume that, in your initial note, you listed the children of Charles and Jane in birth order.
They were James H, Charles and Annie.
My previous note shows that I found 'a' Charles Nankervis (26) with wife Jane (22) at St Just with a 1 month old son named James H Nankervis.
Annie was obviously born later so would not appear in that Census.
Now, although ages in Census records often vary, Charles would have been born about 1864/5 and Jane about 1877/8 with young James obviously born in the Census year of 1891.
You also state that the wife of Charles was Jane GRIBBLE and, as in my previous posting, I have not found a marriage for Charles Nankervis to Jane Gribble at St Just or Pendeen.
I checked the 1901 Census and found the following:-
At Carnyorth, St Just, (Part of Pendeen)
Jane Nankervis, married, age 32, caretaker public board school, bn. St Just
Charles Nankervis, son, age 12, bn. St Just
James Nankervis, son, age 10, bn. St Just
Annie Nankervis, daur, age 8, bn. St Just
These seem to fit your description from your initial note so I decided to have another look at the 1891 Census.
You see, your initial information indicates that James was the eldest of the three children yet you mentioned (what seems to be the above) in a different order.
What I wanted to find from 1891 was the location of son Charles and I found something that might throw a spanner in the works.
The only Charles Nankervis I could find of the right age was enumerated at Trewellard Cliff, St Just as the GRANDSON of James and Honour NEWTON.
It is possible that Jane was married twice and widowed at an extremely young age but ...
Please supply all the relevant information you can so I can work on this further for you.
What I have found so far will just keep me chasing my tail so I will await more.
Ian