What is the theme of "Piping Down the Valleys Wild"?

The main theme in this poem, originally titled “Introduction to the Songs of Innocence,” by William Blake, is of innocence and childish happiness. There is a child sitting upon a cloud, which calls to mind the image of a cherub or cupid in a renaissance painting. This child is “laughing” and tells the speaker to play and sing songs about lambs and happy cheer. The piper “piped with merry chear” and “sung the same again.” The child weeps with joy upon hearing these songs and then entreats the speaker to write these happinesses down in a book so that others may read. The speaker then fashions a pen out of a reed and writes these happy songs down so that “every child” may “joy to hear” (enjoy hearing them).
The language in this poem is simple—there is not dense imagery or a complicated plot. There are, however, two potentially religious allusions: the child on the cloud and the “song about a lamb.” William Blake had complicated opinions about organized religion; he rejected the orthodoxy of Christianity and created his own mythology to describe what he saw as the divine creativity inside humans. The child on the cloud and the lamb could, then, both be seen as allusions to the innocence of a Jesus figure without being explicitly religious. I have linked to an electronic version of this collection so that you can see the other poems and illustrations that accompany this piece.