The secret or "funny" word to say to the little girl to unlock a new wallpaper for the Pokemon box varies according to the player's game and training ID. The player will need to log on to a website dedicated for generating the code word for each game and training ID. Once the code is generated, the player will then need to tell it to the father of the little girl while playing the game, and the father will generate a new Pokemon box wallpaper for the player.
The father of the little girl in Rustboro City is Mr. Pepper, and the little girl is Walda Pepper. While it does not specifically say that the little girl is sick, the father says that Walda needs to be cheered up. Walda's house is located to the north of the Poke center. The little girl is on the second floor on the bed, surrounded by Pokemon stuffed dolls with Mr. Pepper standing over her. Most players miss the hidden bonus because there appears to be no indication that there is something hidden in your interaction with Mr. Pepper.
The secret word is not a word, but a code that is unique to each game and Training ID. To get the wallpaper code, the player must log onto the site where the secret word generator can be found. The player's unique code will be generated after he provides his game code and Trainer ID.