This poem is meant to be a lullaby sung by a mother to her child. The accompanying plate depicts the mother watching over her newborn under she soft shading of a large tree. Just like the last word in the title, "song," and the simple 'aabb ccdd eeff ...' rhyming scheme, it is fair to assume that Blake intended this lyric to be sung.
On a deeper level, Blake is reaching out to glorify the natural world, within which we live, in the face of a more subjective heaven, where the Gods reside. There is a revealing humanistic 'heavenly' innocence and purity that the mother praises in the child who holds "secret joys and secret smiles." Notice how the "infant's smiles and wiles" is capable of beguiling both heaven and earth alike.
"Cradle Song" is often praised as a perfect balance between thought and emotion, capturing the mother's joy and love for her child while at the same time expressing her largest fears and meditation on the child's future role in man's world. The ending of the fourth stanza is perhaps the strongest example of the mother's fear, certainty, and gloom that one day her child's "little heart [will] wake" and when that happens, "the dreadful lightnings break." Blake's repeated theme of passing from innocence to experience is obvious in this sweet lullaby.
The speaker is at peace with her child and situation at the beginning of the poem. The personification placed on "little sorrows" who "sit and weep" during the night is representative of the peace found in night, the period of innocence in Blake's constant-running imagery of oppositions between night and day, and suggests the passing of the child from one world (harmful earth) to the next (safe heaven). But notice when morning breaks in line 10, the tranquility is "stolen" from the child, and "cunning wiles" begin to creep into the child's heart (the mother even refers to the morning as "dreadful").
Overall, the poem can be read as a metaphor for the mother's awareness and inability to alter or stop her child from growing up in this world and losing all of his/her innocence. The first half of the poem is a snapshot of how peaceful and joyful the sleeping babe is, but "youthful harvesting" is inevitable, and the mother is left saddened at the fact that while her child may "beguile both heaven and earth" at the moment, it is only a temporary serenity, and it will not be long before all purity and innocence is lost.