These are the most adorable shoes ever and the fit is true to size. My daughter loves bunnies, so she was tickled pink with these!
It is always recommended that you size up 1 full size than what your child's foot measures at. My daughter is 6 yrs and 8 months old. Her foot measures a 12.5 and she normally wears a size 13 shoe... so I ordered her a size 13.5 and they fit perfectly. Ballet slippers are supposed to be a little roomy in the front... at least  half size..  or a half of your thumb print worth of room.
If in doubt, order a half size larger than their shoe size. They can always be exchanged.