I have been making my 5 year old slippers for three years now. The pattern is from the blog "From and Igloo." I would give a link but it seems the blog is no where to be found anymore (if anyone knows where it is I'd love to give credit.) The pattern and tutorial were for sizes 2 and 4 (left to right in this picture.) This year I graded the pattern up to a size 6 (far right.) As you can see from the worn off stitching and button eyes they are very popular and are worn constantly.
If I remember correctly Christine from "From and Igloo" based this pattern off of some of those great toddler shoes with elastic casing in the front and back. This is what my daughter loves about them. They don't fall off. I always add leather to the sole for grip. The leather is all from a coat I thrifted ages ago. One other note about the leather, they make my daughter quiet as a mouse and she can sneak up on us quite easily (that is if we don't hear the breathing.)