I made the Quiltsmart smart bags for all of my grandkids. I even made one for my grandson with John Deere fabric so he could haul his tractors around. The bags take two fat quarters and are fast and very easy to make and are the perfect size for kids.
I searched and finally found the pattern here,
http://www.quiltsmart.com/khxc/index...f=fun_smartbag
You can make the bags with or without the original interfacing. If you want to make more bags with interfacing you can use use plain interfacing from the fabric store.