These run a little small, which I'm grateful for because they are for my daughter for dance and she is tiny! Thankfully they fit her without being too excessively big.
My daughter is a very tall 4 year old who wears a size 5 and the smallest size of these fits her without too much excess material.
Edited to add: Definitely buy an extra pair. They may be durable, but not for a little one! In just two times wearing them, she's managed to get a hole in them! I'll be buying them again because they look *amazing* and fit her with room to grow!