Piping down the valleys wild
Piping songs of pleasant glee
On a cloud I saw a child.
And he laughing said to me.
Pipe a song about a Lamb;
So I piped with merry chear,
Piper pipe that song again—
So I piped, he wept to hear.
William Blake uses lamb as a symbol of purity. The title Songs of Innocence is exemplified by the use of Lamb, particularly a representation of Jesus Christ. Lambs are considered docile, innocent and appear frequently in the bible. Blake develops this in his song,  The Lamb . 

Introduction to the Songs of Innocence