I know it is not really what you want to here

but going by what you say, it does sound like those 3 children born after 1861 were born out of wedlock. If she is listed as a Widow in 1861 it would be pretty hard for her to conceive

. I would try and get hold of Joseph Cantle's death certificate to make sure that he did actually die (and they were not simply estranged).
I would say though that if the children do not have fathers listed on their certificates the children have probably taken their mothers married name so it sounds like they were legitimate but actually they were not.
.............I will have a dig about over the next few minutes and see if I can come up with anything to post here on this message.
Found this in 1881:
Dwelling: 2 N Garrel Row
Census Place: Llangynwyd Higher, Glamorgan, Wales
Source: FHL Film 1342285 PRO Ref RG11 Piece 5336 Folio 61 Page 15
Marr Age Sex Birthplace
William CANTELL U 36 M Maesteg, Glamorgan, Wales
Rel: Son (Head)
Occ: Haulier C Mine
Letisha Elizth. CANTELL U 38 F Maesteg, Glamorgan, Wales
Rel: Dau (Sister)
Occ: C Mine
Amelia Maud Mary CANTELL U 34 F Maesteg, Glamorgan, Wales
Rel: Dau (Sister)
Occ: Lab In C Mine
Amelia Brittania CANTELL U 19 F Maesteg, Glamorgan, Wales
Rel: Dau (Sister)
Occ: House Keeper
Stephen CANTELL U 15 M Maesteg, Glamorgan, Wales
Rel: Son (Brother)
Occ: Ironstone Miner
Found a death for a Letitia Cantle (free bmd):
Deaths Mar 1895
Cantle Letitia 56 Bridgend 11a 542
These were the only death I could find (free bmd):
Surname First name(s) Age District Vol Page
Deaths Mar 1840
CANTLE Joseph Bristol 11 147
Deaths Dec 1850
CANTLE Joseph Bristol 11 142
Deaths Jun 1858
Cantle Joseph Keynsham 5c 506
This last one is odd though, I came across a death of a Hannah Cantle also in Keynsham, her birthyear would be out though (this one was born around c. 1787)
Surname First name(s) Age District Vol Page
Deaths Mar 1874
CANTLE Hannah 87 Keynsham 5c 549
The district Keynsham spans the boundaries of the counties of Gloucestershire and Somerset, If I am correct Hannah was born in the Gloucestershire region and Joseph was not born in Glamorgan, so his orgins may also be in Gloucestershire