Pattern Notes from p2Designs.com:

“adjusting for other sizes: following the pattern as written results in a men’s size 9 - 12. finished slippers should be between 10 and 11 inches in length. the pattern can easily be made larger or smaller by varying the number of stitches. (the total number of stitches cast on must always be an odd number), weight of yarn, and needle size.

for children’s, try size US 10 needles and either one strand of a chunkier yarn or two strands or sport-weight yarn with a cast on of 19-21 stitches.“