CrazyLittleFingers is a simple application that disables the screen, somewhat like a screensaver, but makes pressing the keys generate different pictures or sounds, probably entertaining the kid enough that they’ll smash your keyboard and drool all over it.
Cat proof the entire area of the computer center with a can of air–easy, we don’t want to hurt the cat–but once or twice is enough, a couple of quick bursts.
Kid proof the keyboard by lightly wiping alcohol across the keys, etc. The kid will find he/she has a horrible taste in the mouth when sucking on fingers-thumbs and suddenly conclude that the source of bitter taste comes from the keyboard.
Simple Pavlovian psychology!