Hi Amanda and Welcome to the site.
I have had a look at this problem and it is not difficult to see why you are stuck!
There are a number of things that are confusing matters including the South Australian Passenger Lists, the ages of Thomas and Mary as recorded in 1841, the number and names of the children and also the names of Mary Caddy's parents.
And none of this is helped by the fact that the family left Cornwall before the 1851 Census!
The site 'Passenger Lists South Australia' shows three different Thomas Hoskings arriving in South Australia with two of those have a wife named as Mary.
It appears your family is the one with the wife mistakenly identified as Mary JAMES!
Thomas Hosking and Mary James were married at Lelant in 1830 and had quite a number of children through the 1830s and 1840s. (But they have not helped either as they also seem to disappear by 1851.)
Unfortunately that South Australian site contains many such errors.
And I think the reason is that those adding the information are quoting from works (e.g. family histories, historical biographies) compiled many years earlier. Those compiling the earlier works had the same problem that many of us have today which is that there is still a lot of information (Parish Registers) not yet readily available.
The Thomas Hosking/Mary Caddy marriage took place at Ludgvan and from 1813 onwards it is mostly only baptisms that have been available on IGI with only a scattering of marriages. Hence the marriage to Mary James was 'claimed' because it was the only one that seemed to match the other available records.
You stated that Mary Caddy was 'born' at Gulval in 1813. She stated in 1841 that she was age 29 so I suspect she was actually born about 1811 or perhaps 1812. 1813 was when she was baptised.
Based on those details she does 'appear' to be the only possible Mary Caddy but it is the names of her parents, Robert and Joan, that are aiding the confusion and uncertainty given that neither name appears among the children of Thomas and Mary Hosking.
So the first question is - were there any other children born after the family left Cornwall?
The next thing causing some headaches is what appears to be another child that was previously unknown to you.
James son of Thomas and Mary Hosking bp. 28th July 1833 Ludgvan (born 23rd June)
This is undoubtedly the same family and the next child, Mary, was not born until 20th December 1834 and baptised 4th January 1835. This then meant that there were two sons given the name James which opens the logical possibility that James was the name of Thomas Hosking's father. But two things are upsetting the applecart here:-
1. I can find no baptism for a Thomas Hosking son of James anywhere in the right time-frame.
2. There is another entry in the Ludgvan Parish Register that makes it unclear whether the child was actually James.
Given the James you know was born 23rd April 1837 and only one James appears in the 1841 Census it means that this earlier child must have died pfior to 1837. And now comes the little problem!
The only burial I can find is:-
Thomas Hosking of Newtown age 11 mths buried 5th February 1834 Ludgvan
The only family this child could belong to appears to be Thomas and Mary and Newtown is where they were living when the next two children, Mary and James, were born.
But to be age 11 months in February 1834 means that this child would have been born about March of 1833 yet the baptism for James clearly records that he was born in June 1833.
I am sure this must be the same child and I suspect that James will be the correct name but it may not be possible to prove. The age problem is understandable given that 'seven' and 'eleven' can sound quite similar. Perhaps 'seven' was the age given to the Vicar but he misunderstood and wrote 'eleven' instead.
IF that child was really James then the argument for that being the name of Thomas Hosking's father is quite reasonable but unfortunately no match can be found. Somehow we need to find some more information and clues.
I think the best thing you can do right now is to provide me with ALL the information you possibly can about the family once they arrived in Australia.
I don't mean down to the present day but certainly everything you know about them up to about 1900 might be helpful.
When and where did Thomas and Mary die and what information is provided?
Do you know of any other relatives in Australia around the same time?
Any other children born in Australia along with details.
Anything at all that you can think of might provide a clue.
CT
And none of this is helped by the fact that the family left Cornwall before the 1851 Census! 
