What is the exact theme of A Cradle Song ?

I think the main theme has to do with Christ's representation in a baby. The baby is pure and not yet polluted by human folly. The mother can see the "Holy image" of Christ in her baby's face. Jesus was a "Sweet babe once like thee" who in His turn wept for the mother. In fact, He "Wept for me for thee for all,/When he was an infant small." In the innocence of her baby, the mother can see the beauty and significance of the Incarnation.

The poem ends with a parallel between the baby in the mother's arms and the Child Christ. Whereas the baby's smiles beguile the world, the smiles of the baby Jesus "Heaven & earth to peace beguiles." The baby is a tiny representation of the cosmic significance of the Incarnate Christ.