Hello
Have you tried to get a birth certificate for one of the other children? I found several on the 1861 census

John Jones age 13 born Myydfai, Carms ?
Thomas Jones age 10 born Ystradgynlais
Margaret Jones age 8 born Ystradgynlais
David Jones age 4 born Ystradgynlais
William Jones age 2 born Ystradgynlais
RG9 Piece 4089 Folio 17 Page 27
or Daniel age 9 at the same address in 1871
RG10 Piece 5434 Folio 8 Page 7
A Baptism record probably won't give Magaret's maiden name so you either need her marriage certificate or details from the Parish Register, or a birth certificate for one of her children.
Not sure whether you know that Ystradgynlais is one of those places which is on the border between 2 counties, in this case Breconshire and Glamorganshire, so the birth may have been registered in a different county. You have enough information from 1861 to try to get the birth certificate for William ~(address, parent's names, David occupation, etc).
modified - College Row was in Gurnos Parcel of Ystradgynlais. Registration district would have been Neath, Glamorgan
Best wishes
Morgan