Cummings is calming a child at bedtime, whose child problems and worries have made him/her restless.  At the same time, of course, Cummings is calling us all "little children" who are worrying about little, everyday things that seem important to us in the moment, but when compared to "large" things lose their power and become less fretful.  The narrator of the poem then lists some "big" things, natural phenomena (rain, snow, sun, moon), and "prays" to them, asking them (their "spirit"), their "bigness" to enter us.  Cummings spent his lifetime doing essentially this same thing in all his poetry (and his prose "The Enormous Room") -- telling the reader that the ordinary, everyday fears and frets of human beings can be done away with by observing and appreciating the real "Largeness" around us.  I recommend "Maggie and Millie and Mollie and May" and "Epithalamion" as further examples of this message to us.