I've always set up a restricted account for my son (who is now 7 years old). It has the password disabled, so he just needs to click his name/icon on the sign-in screen. He doesn't need to install stuff himself, he asks me or his mother if one of his games isn't already installed.
And like yourself, my child is not allowed to use the computer unsupervised.
edit: by the way, my son absolutely loved playing with the
Playskool Store. He would spend hours playing with this - it was one of the best Christmas presents we ever bought, and I'd highly recommend it for any parents who have children who enjoy playing with computers.