British toy designer Patrick Rylands created the Slow Truck in 1972. It was made for Creative Playthings by the German company Schuco. There was also a Slow Bus, which looks more like a bright red Slow Winnebago. Both toys are almost brutalist in their rugged, monochrome design, and when you wind them up, they just lumber along.
I'd check with Tank Girl creator Jamie Hewlett to see if he played with one of these little rigs as a kid. This particular one's in sweet, unplayed condition, so it's ready for your kid to make his own. And it's small enough that, when he's grown he can take it with him to the new life that awaits him on the off-world colonies.
SHUCO CREATIVE PLAYTHINGS WIND UP SLOW TRUCK MIB, first bit $25+s/h, auction ends Sep 26 [ebay] [final bid was $51, whoa]