Children's rings are the trickiest of jewellery purchases. Our suppliers tell us that our US sizes fit the age i.e. size 2 for a 2 year old. The problem with this, is that children have different sized fingers at the same age.
This is why we're absolutely happy to accept returns for replacement or refund.
Here is a great chart you can use, if you are able to put a piece of string around her finger and measure it. This way you can match it up to the US sizes that we use and it will fit perfectly.
My suggestion though, is to purchase one size up. This way she'll get more wear from her ring as she can wear it on her middle finger first, until she outgrows it, then it can be worn on her ring finger. When she outgrows that, she can wear it on her little finger. This ensures she'll get many years from your gift. It's fashionable now to wear rings on any finger, not just the ring finger.
It's the inside circumference you need to match up to. I hope this is of assistance to ring buyers!