Once in Royal David's is not my favorite hymn, but Luther's sermons for the First Sunday after Epiphany (the obedience of the Child Jesus) are not out of tune with the stanza in question:
The Evangelist implies such things as fit with the household routine and the fourth commandment, thus simply attesting his childhood obedience toward his parents. This we should diligently note, so that we not only know what the child Jesus did in his youth and follow his example, but also that we do not doubt but believe that such works, like obedience to parents, have been sanctified and blessed through the child Jesus. Because he, through his person, sanctified these tasks, we should quietly strive to do the same. But you can't tell the world anything. That is why we have disdained such tasks and obedience and rushed into the monasteries. Everything thinks he can do it better and more excellently than the dear child Jesus. Do they not see that such household chores and obedience to father and mother have been sanctified through this holy person, God's Son, who in his youth gathered wood, picked up shavings, started the fire, fetched water, and did other household chores? Truly we are not worthy even to follow his example. (House Postil I:231ff).
In the context of the carol: Follow His example? No problem. We "must" be as good as He, i.e., perfect? That's a problem.
If Luther is in tune with the stanza in question, why do you think one cannot find it in any of our hymnals that have the carol?
There are two verses that didn't make the cut. The one we've been discussing:
And through all His wondrous childhood,
He would honor and obey,
Love and watch the lowly mother,
In whose gentle arms He lay.
Christian children all should be, mild, obedient, good as he.
And this:
For He is our childhood's pattern, day by day like us He grew.
He was little, way, and helpless, tears and smiles like us He knew,
And He feeleth for our sadness, and He swarth in our gladness.
I'm probably too cynical to ask "why" for I'd say that it does, indeed, speak to law. We don't want to place our children such a heavy burden, perhaps not recognizing that all of us are children. Or, actually, maybe so.
May I ask you why these verses haven't made it into our hymnals?
It's unfortunate, as the carol, in its entirety, tells a story just as all our hymns do. The last two verses of the hymn share the good news that "our Lord is in heaven above, and He leads His children on, to the place where He is gone." As well, "Set at God's right hand on high; When like stars His children crowned, all in white shall be around." If we put the verse in question ("must") in context, it isn't such a bad thing (is it?)!
