Hi Trish, I've found that census. I don't believe that Thomas is directly related to James & Mary Parsons in High Ham, certainly not as one of their children.
I believe I now have all their children from the parish register. Following their marriage in Jan 1783, I've got:
William - Sep 1783 bapt.
James - Apr 1786
Anna - Mar 1788
George - Dec 1789
Anne - Oct 1791
Sarah - Mar 1793
Maria - Jan 1795
Betty - Jan 1797
Harriet - Mar 1798
Claudina/Cloadinna? - Dec 1799
FreeREG seems to agree with this list.
There appear to have been three Parsons families baptising children in High Ham during that period; James & Mary as above, William & Mary, and George & Joanna/Johanna.
So although the father's name isn't particularly legible in the entry for Claudina/Cloadinna, I'm happy it's a J for James, not George or William. I'm pretty sure that no-one could interpret their daughter's name as Christianna. Although you can't see from the cropped image below, most if not all entries on that page are in the same handwriting, and if the "d" in the middle of her name was being interpreted as a "t" - well there's not a single "t" anywhere else on the page that hasn't been crossed like the one in Beth, the entry immediately below, so I find that very doubtful.
I looked at another tree on Ancestry last night, which James & Mary appeared in. There were more than 18,000 individuals in the whole tree. I checked three children attributed as offspring of James & Mary's daughter Susan. All three had different parents, none named appropriately, and each was from a different part of the UK