The best way to ensure the right fit is to measure your child's foot length. How to do it? Simply let him or her stand on a white sheet of paper. Then make a point where the front-most finger is and then at the back of the heel. Check the distance between the two. Then choose the size that is at least 1 cm larger than it.
Example if your child's foot is 11.5cm, choose size 20-21, which is 12.7 inner length.
Your child needs the room inside to feel comfy and to allow for growth. Remember that the closer your child feels to barefoot, the healthier it is for him or her.