"Grow up, kiddos, and learn where some of these characters came from" was part of a topic entitled "Why do people consider ROB to be a WTF rep? WTF, people." created by user MewtwoPlusTwo.
The body of the topic read:
"He isn't a WTF rep. He's part of Nintendo's history. He helped saved the video game industry. Definitely not a WTF rep.
Grow up, kiddos, and learn where some of these characters came from."
This, of course, started a barrage of parody topics involving Dark Pit, Duck Hunt and Ridley just to name a few. Even after the topics disappeared (or were modded), this "kiddo" phrase stuck around and was posted in topics where the user didn't seem to know what they were talking about, or were bitching about the playable characters in the game.