The poem is packed with alliteration, which contributes to its grand, musical style. Listen to all the alliterative words in the first stanza alone:
Never until the mankind making
Bird beast and flower
Fathering and all humbling darkness
Tells with silence the last light breaking
And the still hour
Is come of the sea tumbling in harness [...]
This lush mix of sounds—plosive /b/s, fricative /f/s, liquid /ls, and sibilant /s/s—immediately lends drama and emotional power to the language. Combined with the heavy enjambment, it's almost like an outpouring of song on behalf of the "Child." The alliteration also sounds insistent and emphatic, as the speaker supposedly "Refus[es] to Mourn" in the face of a great tragedy. The broader sibilance of the passage ("darkness," "silence," "harness," etc.) adds to the effect, casting a somber atmosphere over the poem.
Readers can hear this insistent quality in lines 14-16 also, as the speaker expresses a kind of pious outrage at the thought of inappropriate mourning:
I shall not murder
The mankind of her going with a grave truth
Nor blaspheme down the stations of the breath [...]
The alliteration softens in the final stanza, as the tone switches from vehement to hushed and sober. Still, it remains strong through lines 19-20:
Deep with the first dead lies London's daughter,
Robed in the long friends,
Combined with assonance ("dead"/"friends," "daughter"/long"), these /d/ and /l/ sounds make the language almost as dense as a tongue-twister and as musical as a nursery rhyme. The poem begins to sound like macabre children's verse as it mourns a child gone too soon.