Hi Rosie99
William Morris and Ann Morris nee Evans were my Gtx3 GP, through their son George (1840-1921), on my mother's side, and Ann's brother, Henry Evans, was a Gtx3 GF on my father's side. On marriage in 1826, William was ~61 and Ann was ~27-28, and from the christening date of son William in 1826 (who seems to have died young), she was pregnant on marriage.
Like you, I have traced William and Ann's children in later censuses, but not Thomas.
Re. baptisms, as Orielbenfro has mentioned, perhaps the other children were baptised elsewhere. Many of Ann's Evans family are buried at Sardis chapel, including William and Ann's daughter Elizabeth (Allen). According to Pembrokeshire Cenquest, Pembrokeshire CRO has records of baptisms at Sardis for 1803-1837.
You mentioned that you were related via daughter Mary, presumably after her marriage to Thomas Marsh. In the 1861 census, the "son" William (aged

was their grandson, an illegitimate son of Mary (born 21 December 1852). There is an interesting article on William after his emigration to Australia in the late 1880s at the following URL:
(
http://snorbens.wordpress.com/2010/08/page/2/)
Regards
Chris101