The tires on my sons Walmart Powerwheels car are thin, hollow, and make of plastic. They cracked and split over time (I may have contributed to the cracking by driving the car around myself). This adapter fits the stock drive sprocket to a 10" Pneumatic Harbor Freight wheel and tire ($5.99 as of this posting). The bolts that came with the wheels needed to be replaced with longer ones (they are M8 hex head bolts and needed to be ~30mm).
There is an axel spacer inside of the plastic wheels (12mm axel with a 5/8" wheel bore) that is reused with the HF tires (which luckily also have 5/8" I.D. bearings), but a second one is needed for the other bearing, so I created a model of the spacer as well. When everything is put back together, there is about 3/16" of play between the axel bolt and the tire, but it doesn't present any problems.
This drive sprocket appears to be the same one that came with my car. http://www.thingiverse.com/thing:917423
Printed with 4 shells to ensure there was enough strength.