"....stuck the knife in the baby's head...." cruel mother!
Don, sometimes it's six feet long and all. The Clancys told it that way.
The knife isn't "stroked on a straw" though. They don't seem to do the act with the straw on knives - in the ballads. That trick seems to be used on swords of various types.
Lots of examples of it being done outside of the ballads after sharpening knives or razors. I wouldn't be surprised if it turns up in a ballad. Just haven't found it yet.
Cheers, Joy