We're going to go ahead and skip the whole, E.E. Cummings doesn't use capitalization spiel. We got that part, right? Good—moving on. "maggie and milly and molly and may" is also a title that captures the speaker's childlike perspective and tone by using simple rhymes and repeating "and" like a kid would. He could've taken the more sophisticated route and separated the names with a comma, but that wouldn't sound nearly as fun and carefree. Remember, we're at the beach, and we're kids, so the last thing on our minds is sophistication via punctuation.
Alliteration does its part by linking these four girls together in a way that visually makes them look similar in a way. We understand that these are separate girls with separate experiences, but the title also suggests that they have something in common with all of those M's. What's that something they have in common, you ask? Well, we might imagine that these girls, just like everything under the sun, have the sea to thank for their existence (or, at least for the chance to reflect on their existence). They go to nature to encounter some part of what it means to be human and, luckily enough for us, we get to tag right along with them (free of charge) for the same sort of reflection.