My kid (5 years old) start to create it own level with Secret Maryo Chronicles ( http://www.secretmaryo.org/
He is creating a lots of level. Now the mouse, keyboard has no secret for him. He is selecting multiple object, copy/paste and move them around. He also learn to save often in case of a crash... Or when he pasted ~12000 life and overkill the system. He learn to take stuff from pre-made level to create his own. He figure out by himself to change the path of the element to modify the images.
He wanted me to change few items colors... But when he ask me to create more enemies I told him to go to school so he can learn!
But three years old is a bit young for the computer... And some key shortcut are nasty and they can click everywhere...
Give your kid a IPod to play with, touch pad will probably be the future peripheral. You might see a video of it own.