That's a good prank to play on colleagues with Macs. Your little one must be very devious!

Here's the solution: http://docs.info.apple.com/article.html?artnum=107329

"You can toggle the display between positive and negative by using the Control-Option-Command-8 key combination. First, press and hold the modifier keys (Control, Option, Command). While holding the modifier keys, press 8.

The screen will go blank for a second and return in the opposite mode."