Reginald Heber wrote this song in 1811. He definitely wrote "sons of the morning", not "suns", tho the change is surely natural in context
("Dawn on our darkness"). One source, http://nottinghamchurches.org/hymns/brightest.html, says that the phrase alludes to Isaiah 14:12. If so, the puzzle thickens, for that verse refers to Lucifer, a *fallen* angel commonly identified with Satan. In any case, the stanza is clearly about the Star of Bethlehem, and AFAIK there is no warrant for identifying that with an angel, fallen or not.

"Suns of the morning" seems plausible to us in that in modern astronomy the sun counts as a star, so that "suns" is available as a poetic word for "stars". Whether that would also come natural to a clergyman two centuries ago is an amusing question, on which the OED, remarkably, casts no light; offhand I would guess not.

Merry Christmas, anyway. %^)