I can only find two likely contenders for the sons Thomas and John in the 1851 census. Thomas, aged 16 and born Holyhead, is listed as living with Mary Griffiths, widow, 66, farming 28 acres at Penial fin-mer (looks like), Llanfwrog. Relationship to Mary is grandson. There are two other grandchildren living there (not John) and a married daughter called Margaret but too young (28) to be Thonas's mother.
In the same year a John G is shown as living at 58 Mill Street Holyhead with his grandmother Anne Williams, 77, Pauper. He is shown as, born Holyhead and a scholar pauper. Could these be the right ones?
Wynne