With a bit more searching, I found that the Welsh name of "I saw a tiny streamlet" is "Y Ddeilen ar yr Afon" (The Leaflet on the River), and is contained in John Owen's Gems of Welsh Melody, second series. A poem of that title can be found online at Google Books (click), though I can't verify this is for the same song or tune. I haven't found a score, nor do I yet know where the work appears in How Green Was My Valley.