Hello Pat.
I looked up the baptisms of the two Mckay children born in Llanferres.
Feb 16 1879. Robert Duncan. S/O James & Jane Mckay. Abode, Nant Plymog.
Fathers Occ. Game Keeper.
Aug 7 1881. Margaret


?. D/O. James & Jane Mckay. Abode, .
Fathers Occ. Game Keeper & Bailiff.
Margaret had a second name but I (we, I asked other people in the search room for their opinion) couldn't make it out.
Does Henderson ring any bells?
After all that I forgot to get their abode which might have helped Barry pin point them. I'm sure it wasn't Nant Plymog. I'll look for it next time.
Paul