To best help you choose, here is a small table. 

Refer to the age but especially the height of your child. 

The models, in general, have a loose & loose cut, they fit quite well. 

For example, my Hector wears size 3-5 years & he is 4 years old & wears 110 and 116cm. Victoire wears size 18 months or 1.5-2 years for her 18 months, she is generally 92cm.

If you are unsure, contact me by Dm or Mp.