Not to put too fine a point on it, but I've been listening to the Kingston Trio's version of this song for 40 years and the line is not: "And the mothers called, "Mary, call home your child," their eyes all drowned in tears", as Joe Offer states, but:
"...call home your child, 'ere ours all drown-ded be".

Just saw the timeless trio last night in San Francisco. Sadly, they did not play this song, despite my calling out for it numerous times. I think some people get distracted by the unexpected element of this tune and do not appreciate its moral message of the Golden Rule. Also, it is rather sweet that the young Jesus in these lyrics is innocent of the fact that all children can't cavort across sunbeams, as He can.