I ordered two pair of these: one for my two-year-old and one for my four-year-old. Both ran quite big. I was able to get away with putting socks on my younger daughter, but had to exchange for a smaller size for my older daughter.
My daughter is size 8 for normal shoes. I ordered a half size bigger (8.5M) because her dance class doesn't start till 2 months from now. Still, I got a bigger size and they fit just right. I would have her use them if her class was starting right now. So I'm getting a refund and I ordered size 9M instead.