electron said:
This looks like such a cool toy:

Hopefully they will make these someday.
It should be very easy for someone to turn this into a product. Line-following robots are pretty easy. If the annotation is just marks (stripes or dots) along the side of the path, the sensors can pick this up easily.

Line-following robots are a standard example that people work with when they are learning about/building robots. The hardware is pretty straightforward.

I think the only things that need to be considered here are scale (and the toy manufacturers should have no problems keeping things small) and speed (i.e., how fast the car moves). The cars need to be kept small so that you don't need a huge area available for a large piece of paper. You'd probably want to be able to run on a poster-sized piece of paper.