Lego Star Wars is my daughter's favorite game as well. My cabinet has 4 buttons per player and that is the minimum number for controlling your character (1-Attack, 2-Jump, 3-Force, 4-Tag another character). To fully complete the levels you will also need an extra button or 2 to cycle through the list of available characters. I wound up using the 'coin' button for this purpose since it wasn't being used in this game.
-Steve