Thanks to
juno_magic, who just knew who to ask, I now have Toddler Key on my computer, downloadable
HERE or
HERE.
Description of the software: Toddler Keys is a useful tool for parents that allows you to lock your computer keyboard, CD drive doors and power-off button. When the keyboard is used it will display images and play sounds every time a key is pressed, thereby preventing access to the desktop and applications, while adding some entertainment value for the kid. You can select the images and sounds to be used by copying them to the Toddler Keys folder. To exit the locked screen, just type the word QUIT.
I've intalled it this morning, and it seems to work.