Kid Key Lock prevents accidental keyboard, mouse input
It's annoying for kids as well. I can't count how many times I've been watching my son accidentally bump a button and cause his Flash-based game to lose focus.
Kid Key Lock is a free application that disable sets of keyboard keys and lock down mouse buttons and actions (like wheel movement or double-clicking).
It's not fully customizable, but it allows disabling keys by groups: none, all, just system keys, or all but space, enter, and characters.
In case you accidentally lock all keyboard and mouse functions, the program supports a password - which you can enter to override the lock or enter setup or quit the app entirely.
Kid Key Lock is also handy for when you want to give your keyboard a quick cleaning without shutting down first. It's a free program, and runs on both 32 and 64-bit Windows.