Hi Tony
Forgive me if I'm repeating myself but in my opinion there are at least 2 ways in which the text of 21 could be linked in with the chorus of 20.

We know pretty conclusively that someone in Scotland in the late 18th century was deliberately mixing the 2 songs up. However this was not in the form shown above so I think this way unlikely.

The second way is much more obvious and simple. The 2 songs cover the unusual subject of new-born baby murder. I can't off-hand think of any other old ballads that deal with this subject in English. Couple this with the well-known hybridising of songs by the traveller community, among others, and you have your reason.