I have last years Disney Run model and had to go up 1/2 size with New Balance versus my other shoe brands.
I had a friend buy my son this years model, she bought him a youth 4 and they were huge on him, luckily I found someone else who had the same model but smaller size 3 to exchange with and they are still slightly big, but not as bad.
My advice go up 1/2 size if you can or at least go to a shoe store and try on a pair of new balances to get a size!