Consider this. About half of the covers of What Child Is This? that I have heard within the last five years have changed “ox and ass” to “ox and lamb.” What would William Chatterton Dix think of this? We shall never know; but in my humble and sometimes wrong opinion, it is presumptuous to arbitrarily alter a word in a 150-year-old song for fear of offending someone with the word ass. I will grant that different hymnals often have different versions of songs. In fact, Wikipedia lists variations on nine lines of What Child is This? based on five hymnals. It is interesting, though, that none of those alternatives includes “ox and lamb.”
Here’s the nub. As an erstwhile songwriter, I can’t hear that song anymore without listening closely to that line to see what the performer has chosen to do. I can say with certainty that any good songwriter (and Mr. Dix was one) would not arbitrarily pick “ox and ass.” He picked those words because those were the ones he wanted in the song and we may reasonably assume that he thought about using lamb somewhere in that song. Mr. Dix knew that lambs are found in a fold, while oxen and asses are found in a stable, where the Son of God was born.
You may say that the presence of shepherds in verse one justifies the reference to a lamb in verse 2. This is a good argument, but I would counter: First, the songwriter indicates that these two are feeding; if a lamb did follow his shepherd to the stable, it would not be his normal environment for feeding (I’ll submit my judgment there to anyone who knows more about sheep than I do). Second, and most importantly, changing a super-classic song out of fear is simply a bad idea.