Hi Mrrzy,
You were quite right to point this out and it is relevant. The thought had crossed my mind once or twice when listening to the song. Off the top of my head I can't think of any other ballads where the 2 motifs combine except for this one and The Cruel Mother so it could just be that the image of boys playing at ball was a common one and the looking over the castle wall is a widespread commonplace in ballads. However the English broadside ballad, The Cruel Mother (Child 20) is late 17th century and in Scotland it acquired stanzas from Child 21 during the 18th century, not found in any English versions, so the lifting of an image from Child 20 into our ballad here would not be at all surprising. The Gentry in Scotland and later Scots editors frequently moved material from one ballad into another so it was accepted practice. Personally I think many of the so-called commonplaces were the result of this mixing and matching rather than oral tradition.