To start with, you will need a POWER WHEELS.
The one I am using is the JEEP COMMANDO. It's a simple, boxy vehicle with a steel frame under the plastic body.
Instead of spending good money to buy a POWER WHEELS, first check at rummage sales, classified ads, and Craigslist. Used POWER WHEELS usually go for about $50, and it you really look around, you can get them for free.
For example, I got mine for free by placing an ad on Craigslist for "WANTED: POWER WHEELS, working or not..." I originally intended to modify it for use for adult POWER WHEELS Racing
, but ended up helping out on a friend's entry instead. (The Jeep DID get used as a pace-car!)
Often, chargers get lost, and batteries are not taken care of and go bad. At that point, many owners are happy just to get rid of the car for free. So, look around, you might now even have to spend any more than the legwork to get your base vehicle!
Not all these cars are the same. Some have more or less structure under the plastic body. Look for one with a good steel frame, that will be easy to attach an overhead rack to.
Chances are that it's been sitting around getting dirty. Help your child take ownership of the project by having him or her wash the car. You may just end up getting more wet than anything, but kids can do more than you think they can - at least let them try!