Little lambs, the sun is setting, little lambs, little lambs; time to rest and stop your fretting, little lambs.
Little lambs, when you are sheep, there'll be time and time to weep; shut your woolly eyes and sleep, little lambs.
On the surface, "Lullaby" is exactly what it claims to be: a soothing song to settle young lambs at sundown. The speaker repeats the phrase "little lambs" like a rocking motion, coaxing them to stop fretting and go to sleep. The rhymes are simple and the rhythm is steady, so the poem feels calm from the first line.
The second stanza adds something quieter and a little sadder. The speaker tells the lambs that once they are sheep, there will be "time and time to weep." The comfort turns out to carry a hint of warning. Childhood, the poem suggests, is the short stretch when you are allowed to rest without worry, and adulthood will bring plenty of sorrow later. The speaker never dwells on this or sounds bitter. The tenderness stays in place, which is what makes the poem linger. It reads as a lullaby sung by someone who knows more than the listeners do and wants to give them one more peaceful night.