5 year olds don't care about cars, but give 'em something cool to have a blast with.
Convertibles, Vista Cruiser station wagons with the glass roofs, VW buses with the glass roofs, or anything with flames painted on it.
They will mostly enjoy waxing it with you. Do it every chance you get! If the car is too big a project, they will loose interest, and you will spend too much time working on the car.
If you buy a glass roofed car, make sure to take them in it on a vacation to FL- do a car show at Daytona, then go to a wild animal park. Make sure it has giraffes. Put the kids in the back with a video camera. The giraffes will lick the salt off the roof of the car, and you will have some AWESOME videos of their 12" long tongues cleaning up your glass taken from 3' away! Ask me how I know- I've got some super 8 film footage still around from over 40 years ago when I was 5!!!
This is my buddy's company:
Jobies Ice Cream Truck
The truck is a 1962 vintage ice cream truck he had restored, including all the original equipment. Jobie is a blend of his 2 kid's names, and they go to events and car shows all over the place. Great little family business, great ice cream, and a great time with the kids every time, PLUS a little income!