I've got a book at home that definitely has some plans, British, from the early 70's I think. I know at least of them uses the frame and running gear of a commercial pedal car. As stated before, you may be able to get hold of a used one (with broken plastic body) pretty easily. I'll get the details for you tonight.
I just had a bit of a search on the Net, and came up with a possible plan from http://www.ubuildit.com.au/kids.html
Most of the results are purveyors of collectibles, but aren't they nice!! Have a look at this, even trailers, planes and a boat!! http://www.matteroli.com/carcollection/pedalcarmenu.htm
and http://www.pedaltractorparts.com/ isn't too bad either.
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