My 1940 Episcopal Hymnal has it as #38: "Here betwixt ass and oxen mild, sleep, sleep, sleep my little child..." and lists it as a French Noel, 13th century, by Gevaert. I remember children's choir long ago... The 1940 hymnal is often available at used book stores- I've hardly ever failed to find one! I know I've seen this carol elsewhere and will look around.