A strain of the earth's sweet being in the beginning In Eden garden – (lines 10-11)
The biblical story of the Garden of Eden is all about the loss of that original, ideal relationship between man, the rest of creation, and God. Innocence, for our speaker, is intricately bound up with his relationship with his creator and with the rest of creation.
Before it cloud, Christ, lord, and sour with sinning, Innocent mind and Mayday in girl and boy, (lines 12-13)
The speaker gets explicit about his concerns. He thinks it would be great if we didn't have to leave the innocence of childhood.