Be of good cheer. The song does not say Jesus never ever cried. The Bible notes that Jesus got hungry, that Jesus got thirsty, and that Jesus wept. Yet Jesus demonstrated extraordinary calm in a storm-tossed boat and before dangerous people. Psalm 50 has the LORD speaking:
Every animal of the forest is already mine.
The cattle on a thousand hills are mine.
I know every bird on the mountains,
and every living thing in the fields is mine.
As a farmboy, I heard cattle get excited and bellow. A cow that is lost, hungry, alarmed, or in love isn't afraid to say so. A lowing cow says, more than most reports I know, that, "All is calm, all is bright." Plausibly, there were cattle or oxen near their manger. Plausibly, baby Jesus found their words quite charming moo'd music.
Every kid should have a cow.