There is a story of a young girl in a Sunday school class drawing a picture of the Nativity. Her teacher looked at it: there were Jesus, Mary and Joseph, an angel, shepherds and sheep … and a very fat man. The teacher asked ‘Who is he?’ and the girl replied ‘Oh, that’s Round John Virgin’.
The line is, of course, ‘round yon virgin mother and child’. In fact, the line is ‘All is bright round yon virgin mother and child’, but most people take too much time and breath after ‘bright’. There are two linguistics issues here, possibly reinforcing each other: misunderstanding the archaic word ‘yon’ (and the meaning of ‘virgin’), and running the /d/ of ‘round’ and the /j/ of ‘yon’ into /dʒ/. (Note that the IPA symbol /j/ stands for the English consonant sound ‘y’. The IPA symbol /y/ is something completely different.)