You know how when you're a kid, some things that are real seem sort of imaginary (like death or war) and some things that are imaginary seem like they could be real (like monsters or magic)? The process of growing up has a lot to do with figuring out what's real and what isn't. In many ways, The Watsons Go to Birmingham—1963 is just as much Byron's as it is Kenny's, and both boys do some significant growing up in the book.
The 1960s were a tumultuous time in America, but the Watsons seem mostly sheltered and safe in Flint. But by the end of their trip to Birmingham, both boys have gotten a healthy (or not-so-healthy) dose of the real world first-hand. Death becomes a reality, and they both have to learn that life isn't all fun and games. Kenny has an especially hard time with this one, but Byron, having finally grown into the role of big brother, helps Kenny understand that they have to go on living even though bad things happen. Yup, that's basically what it means to be a grown up.
Byron and Kenny are better off as kids. They might do foolish things, but at least they're protected from the tragedies of the real world.
Byron and Kenny are better off now that they've grown up a little. Even though they're still young, it's important to understand how the world really works.