Dear Edwin
As you say you found Evan Morgan (age 7) with his parents John & Margaret Morgan and his brother John (age 9) in the 1851 Census at Pantnewydd, Pencarreg in the Hamlet of DOLGWM (not Dolwyn).
Unfortunately, the 1861 Census for Pencarreg Parish is not available. The Pencarreg parish covers quite a large area and also covers Cellan, Coedmore, Cwmann and Llanfair Clydogau which are all within about 5 miles of Pencarreg village itself. Cwmann is within half a mile of Lampeter and Coedmore about a mile, Cellan 3 miles and Llanfair Clydogau 4 miles.
The 1841 Census shows a David & Mary EVANS living in Pantnewydd with their 3 children.
Also
In 1841 at Tynycoed, Parish of Pencarreg (Dolgwm area) (If I remember correctly Dolgwm area is between the villages of Pencarreg and Cwmann)!! There was a John Morgans and a Margaret Morgans along with two boys a David Morgans aged 7 and Morgan Morgans aged 5. John Morgans was an Ag Lab and all shown as being born in the Parish.
As Evan Morgan is 10/11 years or so younger than Rachel (nee Davies), Evan would not be found on the 1841 Census as he wasn't born until around 1844. Like you I found them in 1871, 1881 & 1891.
The 1901 Census shows the following:
20, Aber Houses, Llandyfodwg, Ogmore Valley, Pricetown Glamorganshire
Evan Morgan, Head, Married, aged 57, Farm Lab,Born Carmarthenshire. Speaks English & Welsh
Rachael (sic) Morgan, Wife, Married aged 68, Born Lampeter, Cardiganshire, Speaks English & Welsh
I have two possibilities for Rachel Davies (but no guarantee):
1841 High Street Lampeter (no number or name of house given)
Mary Davies aged 49, William aged 15, Mary aged 10, Rachel aged 9 and Evan aged 3. All born in the Parish.
(next door to them in Lampeter in 1841 was an Isaac and Mary Evans with their daughter Jane (this is relevant as see below)
1851 at Aberamman Road, Aberdare, GlamorganshireIsaac Evans, aged 44, Widower, Master Haulier born Lampeter Cardiganshire
Jane Evans aged 15, daughter, Housekeeper, born Lampeter Cardiganshire
Rachel Davies, aged 22 lodger Charwoman, born Lampeter Cardiganshire
(I know Rachel's age is a about 3 years differnce but it looks like too much of a coincidence - but like I said it's only a possibility)
Hope this helps a little. Oh yes by the way, Llanwnnen is only about three miles from Lampeter, but you probably knew that anyway.
Best wishes & regards, Alltcafan