Kim,
I have checked the 1851 and 1861 entries for the Thomas and Ann families - the older John is correct for 1851, and Joseph is correct for 1861
I've listed them again just to make sure I've read them right!!!! Notice the different spelling of the surname.
1851CADWALDAR, Thomas, h, m, 58, stonemason, MGY Llansantffraid, Brogan
CADWALDAR, Ann, w, m, 50, , MGY Llangyniew, Brogan
CADWALDAR, John, s, u, 24, stonemason, MGY Llansantffraid, Brogan
CADWALDAR, Oliver, s, u, 15, scholar, MGY Llansantffraid, Brogan,
CADWALDAR, Frances, d, u, 12, scholar, MGY Llansantffraid, Brogan
CADWALDAR, Ann, d, u, 9, scholar, MGY Llansantffraid, Brogan
CADWALDAR, John, s, u, 3, , MGY Llanfechain, Brogan
1861CADWALADR Thomas, h, m, 72, Stone Mason, MGY Llansaintffraid, Brogan Cottage
CADWALADR Ann, w, m, 60, -, MGY Llangyniew, Brogan Cottage
CADWALADR Joseph, s, u, 34, Stone Mason, MGY Llandysilio, Brogan Cottage
CADWALADR Thomas, s, u, 27, Stone Mason, MGY Llansaintffraid, Brogan Cottage
CADWALADR Frances, d, u, 22, Dressmaker, MGY Llansaintffraid, Brogan Cottage,
CADWALADR Ann, d, u, 19, Visitor (crossed out), DEN Carneddan, Brogan Cottage
CADWALADR John, s, u, 13, Masons Lab, MGY Llanfechain, Brogan Cottage
Perhaps the enumerator made the mistake or maybe the transcriber. You will need to see the images to find out, but I'm not sure if they are available online anywhere yet.
A puzzle for you..................
Jan