Now after having worked in a shoe shop, I would suggest you dont get one. The only give a rough guide and do not even nesisarily mean that they will fity that sive shoe as even same make shoes have variation in their sizes. A trained child shoe fitter will tell you what shoes are suitable for your child from feeling their feet in a shoe, a measure is just a rough guide. So you are better off getting shoes checked and fitted in store.
To give you a rough guide for checking yourself, when feeling for toes, you want to be looking for a thumbs width bwtween their big toe and the tip of the shoe. And for a width fitting, you should not have a visible gap down the side of the foot and the shoe, but when rubbing your thumb along the side and over the top of the foot you should be able to feel some space between them, not the foot right up against the material. You could always talk to the shoe fitter next time for a demonstration and they can show you what to look for and how to feel their feet.