why not just use the one that your grandson already has as a pattern to make him one.
Or here are a few links to different ones that you could get ideas from.
http://www.csnrugs.com/Carpets-for-K...X-CFK1016.html
http://www.csnrugs.com/Fun-Rugs-FT-5019-96-LAA1064.html
http://www.csnrugs.com/Fun-Rugs-FT-G...1-LAA1058.html