Mother:
Hold L1 and press Up, Right, Left, Down, Square, Square, R1, R1, R1 at the character selection.


Father:
Activate the mother code. Hold L2 and press Up, Right, Left, Down, Square, Square, R2, R2, R2 at the character selection.


Two player mini-game:
Hold Start + Select on controller two after you started the disc.
Controls:
Square = forward
X = jump
Triangle = super attack (full power meter!).


Two player mini-game alternative:
Hold L2 on controller two after you started the disc and rotate the right analog-stick clockwise at the title screen.