No expert on this, but I did have some help from a Welsh speaker; I haven't dared to sing it yet. There are some Welsh language sites which will give you some help. An added problem is that there are apparently regional variations in the pronunciation of the u sound. I was told that Welsh is essentially a phonetic language, so much is said as written. Single f is v, ch is usually soft as in loch,y is sometimes short i and sometimes short u.
This is the nearest I can get to it, but I think a Welsh speaker will be along soon to help us both out, this being Mudcat. It's a beautiful lullaby, and I'd really like to sing it.
huna blentin,in vu munwes
chlid a hin nes ud you hon
bry kee ow mam sin din am danat
carry ad mam sin dan vu mron
nee ka dim am ha ruth (short u) gintun (hard g)
nee na un din ar thi (as in thin) gam
hun an dowel (like towel)an will blen tin
hun an vwin ar vron du vam
I do have another verse.
remember that Welsh is a beautiful lyrical language, so the above attempt at translation is far too harsh; the sounds blend and flow much better than i can show.