Let's assume that from Malcolm in the middle only Francis, Reese, Malcolm and Dewey count since they're the ones in the show (with Jamie included if we're being generous) and another hypothetical where all the kids from Malcolm are included.

This is called The Next Letter is E.

To understand what the music has to do with Dewey, you need a little music theory (sorry).

Regular songs go verse-chorus-verse-chorus-bridge-chorus. In music theory people use letters to describe that structure, like this: A-B-A-B-C-B

So what if Dewey, in that car, got the idea to write a song that went: A-B-C-D?

Well that's how this song goes, and because he's into classical music, it's classical piano.