This 3D printed Race Car toy was created in Tinkercad and printed on a Davinci 3D 1.0 printer. The file was broken up and printed in two separate runs with two different plastics on a single head printer.
Bolts to hold the wheels are 50 % fill minimum, 0.2 mm height.

The wrench allows the child to easy remove and replace a tire.

Video showing the how to is at the link below:
http://youtu.be/FYzaKdQSdbI

Source files are at Tinkercad:
https://tinkercad.com/things/gCa7WEUDLXj-racer-youtube-video

I recommend you print this in two stages.
Chassis in one build and the rest in a second build.
Use 50% fill or higher for the bolts.
I use 0.2mm height and 50% fill for everything and it took about 6 hours for body and 6 hours for the rest of the parts.

All the files are available at Tinkercad so modify it and show me how you improved it.