I was just about to e-mail my daughters with the greeting “Sh’mae annwyl ferched” but on googling I found that translated exclsively as Sh’mae dear “women”. When I back-translated, I was offered “ferch” as the plural of daughters.
So - does “ferch” mean daughter AND daughters and “ferched” only mean women?
By the way, I often use Lleucu when addressing my daughter, Lucy (although she’s not terribly keen). I can’t find a Welsh equivalent for my other daughter, Emily - any suggestions? 
no, merch is always singular and can mean girl or daughter (it could be used as ‘woman’, but ‘menyw’ is better for that).
merched is always plural and can mean girls or daughters (and again, although it could be used to mean ‘women’, ‘menywod’ is better).
O, diolch o galon, Siaron. So the best greeting for my daughters would be “Sh’mae annwyl merched” (without mutation)?
Annwyl usually does carry a soft mutation (except when followed by a Proper Name), so it would still be “annwyl ferched”. However, for the phrase “my dear daughters”, I’d be inclined to say “sh’mae fy merched annwyl”.
That’s a good one. I was going to say that some phrases don’t feel right in other languages and that’s why translating at times is a difficult business, but your translation does the job nicely.