This is my attempt. I spoke only Welsh until I was about 5 years old, though I don't speak too much of it these days - I live in England.

Huna blentyn yn fy mynwes
Hinna blentin unn ver munnwess

Clyd a chynnes ydyw hon
Cleed ah chunness uddew honn

Breichiau mam sy'n dyn am danat
breyechiai mamm seen deen am dannat

Cariad mam sy dan fy mron
carriad mam see dan ver mrron

Ni cha dim amharu'th gyntun
knee chaa dim ammharreeth gun-tin

Ni wna undyn a thi gam
Knee oona inndin ah thee gamm

Huna'n dawel, anwyl blentyn
Hinnan da-well ann-ooil blenntin

Hun'an fwyn ar fron dy fam.
Hinnan vooeen arr vronn duh vamm

Notes: TH is always pronounced like THICK not THIS.
I've used double vowels to show the sound, not the length.
Emphasis is always on the last but one syllable.
WYL (OOIL) is pronounced like OIL, but with an OO instead of O
What I've called VER is pronounced like in EVER but without the R

I hope this helps
Ray