Definitely put Tempest on the list--if you have a spinner.
5-yr-olds LOVE spinning the spinner in one direction continuously, and shooting the whole time.
Another spinner game that's really easy is Cameltry.
You just spin the spinner to rotate the maze, and your ball falls toward the bottom of the screen.
Quantum and Marble Madness are good ones--if you have a trackball.
Rally-X is good for joysticks.
They will probably burn through their fuel really quick leaving a continuous smoke trail, but they enjoy it.
Continuing games like 1941, Rampage, Gauntlet and the like are always good as well.
That way they can eventually make it to the higher levels, regardless of skill.