What cheat codes are available for Bratz(tm) Forever Diamondz(tm) on the Nintendo DS and GBA?

Bratz DS
To unlock the piglet enter the below code on the main menu screen:

Left (control pad), B, B, Y, R

Start the game as normal and go through the cutscene; piglet will appear on the Pick Your Pet screen.

Bratz GBA
In the main menu, press Up, Up, Down, Down, Left, Right, Left, Right. If done correctly, you'll hear a sparkle sound. This unlocks the mini-games.