Anachronism Stew: The pros being kids in modern times. Not only that, but also by the time some of the pros were born, others would've already been teenagers or even adults. NFL Backyard Basics handles this by having the present-day pros turn into kids while they're in the Clubhouse.
Cash Cow Franchise: This franchise was the cause of Humongous Entertainment making a profit.
Seemed to evolve further into this as soon as Atari hijacked the series.
Celebrity Power: The pros, who are always better than the neighborhood kids. Justified if you think about it, since the pros specialize in their respective sports while the neighborhood kids play many different sports.
Conspicuous CG: When the player sprites were changed to pre-rendered images, the spectators in the background were still 2D drawings. Likewise, when the camera zooms in on the game (i.e., when someone's at bat or doing penalty kicks), the players look like cartoons again.
Jack of All Stats: Meta-example—the neighborhood kids are always overshadowed by the pros because the pros specialize in their respective sports, while the neighborhood kids play many different sports. Each specific game also has their own general Jack of All Stats. In Baseball, it's Jorge Garcia, while in the 2001/03 versions it was Sally Dobbs.
Took a Level in Badass: The kids' stats vary between sports, causing kids with awful stats in one sport to suddenly become very good in another. Some also invert this trope, going from being good to awful.
Awesome Music: Invoked and lampshaded in Backyard Baseball 2001/Soccer: MLS Edition/Soccer 2004; one of the Credits Gags is "Click here to listen to the awesome credits song. Oh, and see the Smarty Pants who made this game."
The GBA games, except Football, which has no ending.
Backyard Baseball 2001 and Soccer: MLS Edition removed the rewards for winning, besides getting a picture in the Hall of Fame.
In the Football games on the PC, beating the game as Mr. Clanky's Tackling Dummies gives you a simple victory photo. Justified as this team only consists of robots who probably aren't programmed to ride rides at an amusement park.
Covers Always Lie: Although Soccer 2004 was the last game to feature the kids' original designs, the box art depicted their then-new appearances.
Credits Gag: If you simply hover the mouse over the credits button in Backyard Baseball 2001, Backyard Soccer: MLS Edition and Backyard Soccer 2004, you get some humorous messages at the bottom of the screen.
Entering "Houstons" as a coach name in Backyard Football 2002 adds the Houston Texans as a team name and logo.
In the original Football, the kicker for the Dolphins in season mode is named Ray, the same name as the kicker who wanted to kill Dan Marino (who happens to be in the game himself).
Feelies: Earlier installments came with promotional cards of the Backyard kids and pros. The ones that came with Baseball were an Obvious Beta, however, as some kids' clothes are differently colored, and their pitching and fielding stats are inaccurate and labeled as "Throwing" and "Catching".
Power-Up Letdown: The Chameleon play in Football, which turns your team uniforms into the offense uniforms. It really backfires, as it's still easy to tell what players are yours, and it's impossible to fool the computer. 2-player mode doesn't do any better; you can't highlight the pass icon over the defense, so the power up becomes worthless.
Soundtrack Dissonance: The pros in Soccer: MLS Edition did have unique themes, but they can't be heard in the "Meet the Kids" screen. Instead, the menu song/previous kid's theme keeps playing, resulting in things like a lot of pros using Billy Jean Blackwood's cajun leitmotif.