The poem "Infant Joy" is a celebration of a new life. In the first stanza, the newborn baby, two days old, asserts that he or she is happy. Although she doesn't have an official name yet, her name is her state, namely "Joy." The narrator then blesses the infant with joy. In the second stanza, perhaps meant to reflect the joy of the parent who welcomes the newborn, the speaker uses words like pretty, joy, and sweet to describe the child. The two evidently have a conflict-free relationship so far: the baby smiles, the parent sings. The poem closes with the parent blessing the child with joy.

To interpret this poem fully, readers need to consider the companion poem. This poem is from Songs of Innocence. The companion poem from Songs of Experience is "Infant Sorrow." While this poem represents unadulterated happiness, the companion poem represents complete sadness and despair. It describes the child coming into the world "helpless, naked, piping loud." The mother groans and the father weeps—presumably in sorrow, not joy. The child already seems discontented and contrary with its "struggling" and "striving." No blessings are given, but the child is compared to a "fiend." Instead of smiling, the baby sulks.

Blake believed that "without contraries is no progression." To arrive at the true meaning of a new person's arrival in the world, we must consider the contrary states presented by "Infant Joy" and "Infant Sorrow." Considering the truths presented in each poem will lead readers to a greater understanding of the weighty elation of bringing a new human into the world.

It's a really short poem, so I have provided a link below for you to read it on your own if you haven't already.  It's an easy read.  It is basically just Blake rejoicing over the joy of being in the presence of a 2-day-old baby.  It is 2 stanzas long.  The first stanza appears to be from the perspective of the infant.  He  (or she) declares that they do not have a name, and asks what they should call the speaker of the poem.  Then the infant declares that it feels "happy" and that "joy is my name" and then pronounces a blessing on the person with it, "sweet joy befall thee."  The second stanza appears to be from the perspective of Blake.  He states that he feels the infant is a "pretty joy", and that he will call the baby "sweet joy" and that the baby smiles while he sings, and pronounces the exact same blessing-"sweet joy befall thee"- upon it as it pronounced upon him.

I hope that helps a bit.  Good luck!