Even though the speaker of "maggie and milly and molly and may" isn't a kid, he sure sounds like one. In fact, it seems like he's right there with these four little girls experiencing their adventures in the same way they do. He repeats the word "and" like a kid would, keeps his lines short and (generally) rhyming, and even uses phrases like "play one day" in the same way a nursery rhyme would. His style of figurative language also fits well with his informal tone because it doesn't sound over-the-top when he describes a sweetly-singing seashell and a bubble blowing-crab. Sounds simple enough, right?
At the same time, he flexes his poetic chops now and then when he describes the stone "as small as a world and as large as alone." There's some obvious parallelism going on there (fancy) and a paradox (also fancy) that causes us to pause for a moment and think about what the speaker means by that. He also uses words like "languid" (line 6), which is definitely not so kid-friendly. But these instances of fancy talk are sparse and used only in strategic places in order to further illustrate a particular metaphor or simile. So he's a bit like a kid-friendly modern poet, which may sound like an oxymoron, but it's totally on point with this speaker.
It's his childlike tone that really helps to maintain the poem's casual attitude. It's easy to sound flowing and carefree when you're not getting stuck in super-long narrative lines with big words and big ideas. The simpler the prose, the easier the speaker's voice tends to sound. And that's definitely true in this Cummings poem where less is not only more, it's also necessary in capturing the simple nature of the sea and its mysteries.