Gadget, you say you've not found anything on John and Mary's children's baptisms, but Deidre (above) found Elizabeth's baptism to them in 1813.
"Sorry to be devil's advocate but how sure are you that Elizabeth was the daughter of John and Mary and her surname was Williams?"
Good question! I don't have access to my notes at the moment, and this was an area I reached several years ago. If memory serves (and this link details how I got here - then it was through finding Thomas with his Mum in the 1851 census, and spying the birthplace as Mold for the first time.
1851:
Elizabeth age 38 widow born North Wales Mold
Thomas son age 16 born North Wales Mold
William son age 10 born Liverpool
Elizabeth dau age 8 born Liverpool
But there was nothing for them in 1841 until the Mold entry was found, placing him with John and Mary Williams. A check of Elizabeth's son's William Forsyth's birth certificate showed that 'Williams' was indeed the mother's maiden name. Finally, Deidre's confirmation of the baptism of Elizabeth to John and Mary confirms what I had long suspected.
Wow, that was more difficult to justify than I'd expected!
Cheers!