Here's one that might work and it comes in a range of sizes.
http://www.ravelry.com/patterns/libr...ppers-for-kids
You might also try a search at Knitting Pattern Central and an advanced search on Ravelry.
Also, here's a source for suede slipper soles but I'm sure Googling this or some variations will find more.
http://www.fibertrends.com/product/2...dren%27s_Sizes