That is so lovely! The last portion that they hummed without words made me focus on the melody, and I was reminded of the sounds of the noted 19th century Scots-American composer of popular songs, Stephen Foster, who wrote many favorites of a bygone time:
Beautiful Dreamer
Camptown Races
My Old Kentucky Home
Oh! Susanna
Old Folks at Home
I Dream of Jeanie with the Light Brown Hair
Many of them migrated to the US with my ancestors.
It's just as well that they emigrated . . . my father's distant progenitor was a MacGregor, the crime family of Scotland. They were a one-clan crime wave until the British hanged a bunch, threw a bunch in prison, and outlawed the name. You couldn't CALL yourself MacGregor. So our ancestor made tracks for Virginia, and renamed himself MacGahee. And here we are!
Cosmic coincidence: in WWII, my dad was in the old 34th ID but wound up in the 79th Cameron Highlanders for the duration . . . Anzio, Monte Cassino, Biela, and other tourist spots . . .