Ordered this in a size 9 toddler for my granddaughter's narrow size 8.5 (all shoes at 9 so far fit big).  However, this size 9 was too small for her, so we gave those away and ordered a size 10.

One size bigger (size 10) arrived and fits perfectly.  It gives her toes (1/2 size) wiggle room, whereas the size 9 (1/2 size larger her actual shoe size), her toes touched the tip of the boot and would most likely not be comfortable.

Order one size up.  They only come in whole sizes. If a size 8.5 consider size 10, not a 9 - I believe with the thick, warm, comfy sherpa lining, is what makes the shoe run a bit smaller.