Hi Lucy and WELCOME to this site!

I have spent several hours working on this today with interruptions for other things I needed to attend to. I can now fill in some of the information you are looking for but there are some things that remain a mystery.
The main issue is that I cannot be sure of the parentage of Sarah Polglase although I do now know from Census records that she was the grandaughter of a James Polglase and his second wife Sarah Stephens/Stevens nee Hoskin/Hosking who had been baptised at St Erth in 1800.
Back to the beginning .....................
Charles Rowe was baptised at St Ives (St Mary's Mission Chapel) 24th February 1871 having been born at St Ives 25th March 1869. He was the fifth of eight children to William Rowe and Johanna/Joanna Davis who married at St Ives in 1860.
Charles Rowe was first married to Ann Williams Trevorrow 20th February 1892 at St Ives. There were four children from that marriage before Ann died - she was buried at Barnoon Cemetery St Ives 3rd December 1905.
Charles Rowe then married Hannah Broad in 1906 with the marriage taking place probably in the Register Office at Penzance or one of the non-conformist chapels. The only child from this marriage was daughter Hannah Broad Rowe who was born at St Ives 17th December 1906. I have not found a baptism for Hannah but she appeared in the 1911 Census with her grandmother Joanna Rowe and her birthdate is taken from the 1939 England and Wales Register at which time she was living with her father Charles and stepmother Sarah.
Hannah Rowe was buried at St Ives 30th December 1906 just a couple of weeks after the birth of her daughter.
Charles Rowe then married Sarah Mary PHILLIPS in the March Qtr of 1914 - their son Harold Rowe (your grandad) was born at St Ives 23rd May 1914 and baptised 10th July 1914.
This is where I ran into problems initially because I was unable to find Sarah in the 1901 or 1911 Census!

** I will try not to make this too confusing but ....................
As I mentioned above I have not been able to find a birth record for Sarah Rowe but after some interrupted hours working on the problem from a number of different angles I found the following:-
Sarah Mary Polglase was born at Camborne about 1866 and in the 1871 and 1881 Census she can be found with her grandmother Sarah (sometimes Sally) Polglase living at Camborne.
Sarah (the mother) was born Sarah Hosking and baptised at St Erth 26th October 1800 to William and Elizabeth Hosking. She was first married to Edward Stephens/Stevens at Gwinear 15th August 1829 and had a son William (1830) before she was widowed in 1831.
Sarah then married widower James Polglase at Germoe 18th October 1834 and had four children - Ann Hosking Polglase (1836), William Polglase (1838), Elizabeth Polglase (1840) and James Polglase (1843) - and she was widowed before the 1851 Census.
I have not yet been able to confirm the first wife of James Polglase and nor can I be sure of when or where he died. There are two James Polglase deaths in the GRO Index between 1840 and 1851 but in each case the age discrepancey is too much to be the right man. I do know from the 1841 Census that James Polglase was born about 1795.
Sarah Mary Polglase may have been an illegitimate daughter of either Ann Hoskin Polgoase or here younger sister Elizabeth but she might also have been the daughter of William or James Polglase. The fact that I can find no indexed birth record for her and the fact that she does not appear in any Census except with her grandmother does make positive identification rather difficult.

This should help at least a little and perhaps there might be other members here who might be able to add further information.
CT